All illness could be considered a private issue, but people admit to having cancer, bronchitis, and plenty of other physical illnesses. They do it because it's something they're going through and because they're not ashamed of it.
 
When I mention that I have a mental illness, I do it in part because I want to help erode the stigma. I want people to know that there are intelligent, reasonably "together" people who have mental illness.

People also admit to having a broken leg, having diabetes and being allergic.

There's no reason why having mental disease should be any more shameful than having a physical illness.

As for why people talk about their illnesses there can be several reasons:

• Sometimes it's useful knowledge for others interacting with them: if I'm cooking for you and you're allergic to any ingredients, I need to know about it. If I'm trekking with you and you're terrified of heights, I also need to know about it. In both cases for the same reason.
• Sometimes it's just social. Sharing aspects of our lives is one of the things human beings do to bond and make friends. If something is important to you, then I may know you better if you tell me about it.
• It can help remove prejudice. If you know several people you respect who openly admits to for example depression, you may be less inclined to consider depressed people as "bad people".
• It can be part of the cure: For some mental ilnesses and some people, putting words to their emotions and their experiences can help them heal.

Lots of great answers here.  Here are two more reasons:

• If you refuse to speak about something that is affecting your life, then when you avoid speaking about it, you will appear evasive and act like you're lying or not being honest.  People pick up on this very quickly and it will harm existing relationships and prevent you from forming new ones.  Isolation usually leads to worsening mental illness.
• If a topic is taboo, then there is a degree of shame about it.  Many mental illnesses are chronic or recurrent and therefore something that you have to live with.  This means that you are consciously or unconsciously ashamed of yourself which is usually not good for your mental health.  For instance, it makes you more vulnerable to bullies.

"Mental illness is a private issue ..."

I respect that it may be a private issue for you and for some other people—maybe for many other people—but there's no such thing as a something that objectively is private. A private issue is, for each person, whatever he or she wants kept private.

My habit is to have as few private issues as possible. I'm absolutely not into broadcasting details about myself, because most of them are boring, and I don't see why anyone would care, but when someone asks a question, I tend to be honest and open. I also will admit to so-called private issues if I want to explain something and it wouldn't make sense without that context.

I have an openness policy because I find keeping secrets tiring. They seem to be easy and natural for some folks, but I find it hard to remember who is supposed to know what, who I've told what, and who would rather not know what. It uses up mental energy I'd rather devote to other things. So, aside from a few issues, I'm an open book. Being one makes me breath easier. 

I also have a completely materialistic view of the brain. As such, I see no difference between having a mental illness and a broken arm. I certainly don't see the former as shameful. The brain is a biological machine and machines tend to break. Shrug.

I'm aware of the fact that some people feel otherwise, and that by being honest, I might possibly jeopardize my future. Some potential employer could read Quora and think, "I'm not gonna hire that nut-job!" I guess that's a risk I'm willing to take. For me, the payoff—not having to be exhausted by keeping secrets all the time—is worth it. And so far, being honest hasn't had any adverse effects that I know of.

If I lived at an earlier time—or even in another country (or part of the US) in which mental illness was more of a taboo—I might feel differently. I'm not courageous and I'm not a trail blazer. I'm also not an advocate. It's just that secrets make me tired.

I talk about my experience with postnatal depression because its incredibly important that people are aware of this issue and understand it.

The stigma and secrecy around PPD is still extremely intense, and it shouldn't be. Women should be aware that PPD is very common, aware that only two-thirds of mothers bond immediately, aware that there is help available for them.

So I talk about it. For me, it's a way to claw some good out of the horror that was PPD.

I feel responsible to let people, especially at work, know that I have depression and that it is a chronic illness.
 
You see, I have tricks that I have learned over many years to help myself out of the hole that I have been digging. Top of that list is to get away from it all. So I tell my boss and whatever co-workers I may trust that there will come a day when I will call out sick and not sound sick at all. It may be one day or two but if I don't take the time now, I will take more time later when I end up hospitalized. This honesty is, as Christopher Titus put it, "the difference between 'paycheck' and 'medication at bedcheck.'"

To try and create a little understanding, where before there was ignorance. Ignorance is most definitely not bliss in the case of mental illness, the more information we have, the better.

With understanding, comes acceptance, and that's what we are all seeking; to be accepted for who we are.

Speaking out helps others to speak out, then we can all learn from each other's experience. Sufferers often have a better insight than a lot of professionals, especially the highly intelligent users on this site.

Like others have said, and it's a very important point, so I'll reiterate it too; to try and remove some of the stigma, which is still rampant.

I think the main reason I let people know I have a mental illness is because it has had such a huge influence over the person I have become, because years of self doubt has caused me to harbor a sense of being to some extent a lesser person than I could have been and my belief is this was caused by having bipolar disorder. So it is an excuse, I excuse who I am by letting people know of my troubles, that's the cold truth of it.
 
I spent 20 years with depression, a tough 20 years where I had many ups and downs and was then diagnosed with bipolar and medicated for the first time in my life. Since then 'I' make more sense to me, things from my life are clearer and I've gained strength from knowing that chemicals within my brain rather than my character or personality have detrimentally impacted my progression as a person. So I have been quite open about my condition for close to 10 years now.
 
This assists me in that I can talk to people about having a mental illness, I see how different people react, I hear the surprise from some who wouldn't expect someone with bipolar to work, I hear stories from some about their loved ones and friends with mental illnesses. Talking about it frees me to an extent, and educates me and others.
 
Finally there is the stigma side that others have discussed here, the fear that some of society has about mental illness because the silence about the issue and poor media coverage has turned many of us into bogey men. I want people, like myself, in the future to lead an easier life than I have, I want fewer people to commit suicide, I want families to be more accepting of their children and siblings, I want greater employment opportunities for the mentally ill. I want the world to know that the mentally ill are all individual people with unique problems and skills, that we as a group should be treated individually and appreciated far more than ever before. For those reasons I admit my illness, but I have no expectations that everyone should or will.
 
There are downsides to being open though, some people steer clear of you, it can be a concern if known by your employer and co-workers, it can impact your chances of finding a partner, it can kill a conversation, there are many downsides but ultimately I believe, for me, it is proactive and positive. But then I have far less to lose than others so for me it is far easier.

1) People often share things they are worried about in an effort to feel less alone.  For many people with mental illness, their illness is the most pressing concern in their life, so they feel the need to discuss it.

2) If mental illness makes it difficult for them to perform at their jobs or in relationships, it can have a huge effect on their lives.  If you never hear about their illness, you're missing a lot of backstory to their experiences, so it becomes hard for them to talk meaningfully with you.

3) People usually do not tell everyone they have a mental illness.  If they are telling you, it is in part because they trust you and believe you will not judge them.

Why should I hide it?

It's a part of me that impacts the way that I interact with everyone.

Should black people hide their skin? Should gays hide their sexuality?

Why should I hide my mind?

For too long mental illness has been hidden away and so much pain, misery and discrimination has happened as a result.

Letting the light into the shadows is the first step to help support so many people who really really need it.

As a learning diabled adult, my view is "We're here, we're weird, get used to us".

We lose stigma by openness.

To put a face to the disorder. To lend my voice to better educate and teach others about my illness.

To allow for acceptance or possible rejection early on as to not waste their time with me, or them waste mine.

To answer question's for those who want that to learn about my illness because they truly want to be enlightened.

To erase stigma, to also let them know about other famous minds who suffer from the same disorder.

To educate the possible ignorance of those who have prejudice and preconceived ideas against those who suffer from mental illness.

But mostly to be myself, because its part of me, the good,  the bad, the creativeness, the mania, the person I've come to be.

Let us try this way of looking at it.


First: "know thyself."  (Socrates) Then, as you reveal yourself to others, they will know and respond to you. Some may like and value what you share, and others may not. It has been said (and I agree): "it is better to be rejected for who you are, than to be liked for who you are not."

Take the "mental illness" out of the question for one moment, and step back.

Based on the assumption that one is capable of developing reasonably healthy relationships:

It is a natural progression in any relationship, that, as trust and fondness develops, one will begin to share things about themselves commensurate with said trust.  This is why you do not tell your life story on a first date, and why you are not a stranger to someone to whom, for instance, you have been with for twenty-five years. The issue has little or nothing to do with mental illness.

By our very nature, we will disclose, with time, with trust and with caution. This is how we affirm our relationships. We get closer gradually.

So, now let us put the mental illness back into the question.

Yes.  That is still my answer.

I'm on the Aspie Spectrum, but relatively functional.  If I don't respond as neurotypicals expect, this one piece of information may help them avoid an assumption that I'm:
Uncaring
Rude
Stuck Up
Dislike them
Angry with them

I'm also a HUGE introvert, so while I can and do function around large groups of people it is VERY uncomfortable for me and is VERY draining.  This is because I track EVERYTHING.
Thus my outings around large groups or crowds tend to be short (an hour or so).
If we're shoulder to shoulder (as in an outdoor concert) it might be 1/2 hour.

One on one I'm awesome as I pay attention and tend to make almost anyone feel like they are the only person who matters at that moment (which is generally true for me in that moment)

It is extremely painful to hide something that affects every aspect of your life so significantly.

I want the freedom of taking off the "Anonymous" label and saying to the world, "Hi, my name is [my real name] and I suffer from PTSD following a horrifically abusive relationship."

I would love to tell you who I am. I know you all on Quora would be supportive. I've spoken about my abuse and my PTSD anonymously and 100% of responses were supportive.

People who know me IRL and know about my abuse and my PTSD have had a wide range of responsses - from extreme support (my sister) to complete apathy (my dad) to denial (my mom) to intentionally trying to trigger a flashback (my former roommate when I kicked him out of my house).

I don't want acquaintances to stumble upon this, know who I am, and then act like my mom or dad - which may very well happen. (At least they're extremely unlikely to act like my former roommate.)

One day I hope to have the resilience to put up with people who don't even feign sympathy for the hell I went through and the aftermath I'm still dealing with. I know the majority of my acquaintances will support me. And their support will help me heal.

It will also help to raise awareness. Awareness about PTSD. Awareness about domestic violence. Awareness about women-on-man domestic violence. About transgender-on-transgender domestic violence. All of which need to be discussed. It will be much powerful when I talk openly about my experience.

One day, I'll hit the "make public" button. And when I do that, it will be an extraordinary milestone in my recovery.

I am probably not going to advertise it to customers at work, "Hi I have PTSD and a tad social anxiety, so please don't yell or show violence, and don't get too close or invite too many people in."

However, this is who I am. Outside of work it's going to explain my answers, qualify my opinions, and explain my disappearance when someone writes a reply telling me to eat sh*t and die... (never happened here at quora, but has happened).

This is not going away. I can apologize for it, hide it, etc. Yet, much of this was done over a lifetime. I spent 5 years in counseling to minimize the outward symptoms, and it still came back with enough new stimuli... so this is not going to go away, this will likely be with me forever in either major or minor symptoms depending on how badly the PTSD has been activated. There is no real cure for PTSD as bad as mine. There's no one event to zero in on and work through, it's a lifetime of events. I simply learn to function in society, at least for the most part. There are probably days I should be locked away for my own safety, but I am no danger to others.

Some days I cannot talk about it, some days are so bad I would love to just disappear for a week and not talk to anyone for fear of the symptoms getting worse and showing up.

Specifically because of reactions like yours, that there is something to be ashamed of, a failing of the sufferer's moral fiber or inner strength.
No one looks at a person with Parkinson's or Alzheimer's,  or even many cancers (excluding those like lung cancer in smokers) and says they should hide it, be ashamed of the brain tumor or leukemia. And yet people treat depression and other mental disease as of the patient deserves it or chose it, or could get rid of it if they were strong enough.

I've been diagnosed with chronic depression/major depression/disthymic disorder, etc since middle school. It goes hand in hamd with chronic daily migraine for me- sees obvious that if you hurt every day, you are going to be unhappy, but it is more than that. I've tried almost every treatment out there (for both) and continue to fight the good fight every day.
I have a good job, if exceedingly stressful, doing something that gives me satisfaction. I don't ask anyone to carry me via government support or via personal whining for help. I find a way to get things done, even if it takes a lot longer than it should because of my illnesses. No one offers to help, and so I don't ask.
Don't kid yourself- depression and other mental illnesses are just as real as gangrene in a limb, and potentially more deadly because no one gives a damn. At least with gangrene a doctor will amputate. With mental illnesses,  people just say we are faking it or deserve it and make it worse.  You can't see it, so you don't think it is real. Funny how your tumor is real, even if I can't see it just looking at you, but my illness is fake.  Or you think it is shameful and we should hide in a closet. Cuz thzt helps. Too bad - most of the greatest thinkers/writers/inventors/etc had some form of mental illness.

When I admitted I had a mental illness the reason I did it was because I didn't want to feel ruined anymore. Being accepted and acknowledged as a normal person despite people knowing I was mentally ill was a huge part of my recovery. I had been beaten down and abused by someone who told me that my mental illness meant I was ruined. Being accepted and open made it okay to me. I wasn't ruined again.

I was diagnosed bipolar over five years ago, when I was 24. I just had a three week manic episode.

I have always been open about my diagnosis. I don't know, I was never ashamed about the way my brain functions and that is what it is - a brain that functions differently than the neurotypical brain. I don't take it personally.

I disclose because I want the people around me to know who I am. An ambitious woman with a funny brain. It's not who I am, it's just one facet of me.

Maybe their mental illness is apparent when speaking with them, so they're letting you know such that you'll understand instead of wondering what is their problem.

Jessica Su has written a great answer to your question. I'll add what I can.

I have obsessive compulsive disorder. Things are great now but I've been through a very tough time in the past. It made sense to me why many people would want to hide their mental illness but I never felt the need to do so. There are a few reasons for that.

It was pretty clear to me that I was suffering from a natural condition so there was no point being secretive or ashamed about it. I mean I wouldn't be ashamed if I was born with a physical disability, so why should if I be ashamed of my mental disorder. 

One of the things that helped me get over my problems was support from my friends. Yes, I was lucky enough to have the company of very sensible friends who patiently listened to my problems. And that helped me a lot.

Also, people with mental disorders face a lot of stigma in society. So I do feel the need to spread awareness about it. Keeping quiet is not a great strategy for that.

Sometimes I do avoid talking about it when I know that the person I'm facing wouldn't really understand the disorder. I've had people laugh at my face when I tried to explain my situation. At the time I was out of the worst phase, in fact I was much better, and I did not mind much but I'd certainly avoid such experiences in the future.

A lot of people avoid telling anyone about their mental disorder because of the same reason. Many are ashamed of it mostly because deep down they feel like it's their fault.

I guess most people wouldn't admit to having mental disorders in any kind of interview but I feel very strongly about that. I feel that I am a competent person and I would like my employer to respect me for who I truly am. If they think that merely my history of mental disorder disqualifies me for the job even after I make it to the interview, I'd rather not work for them.

I do it for a few reasons:

• Because talking about it helps me make peace with it.
• Because I want the attention.
• Because sharing something personal about yourself is a good way to get other people to share personal things about themselves, which brings you closer together. I've made a few close friends this way.
• Because it makes me sound like a more interesting and rounded person.
• Because it helps me establish this narrative in my head of overcoming my issues, which, being a white male middle-class person, I haven't had a ton of yet. (The whole bi atheist thing might throw some more adversity my way, but those are both easy to hide.) That storyline helps me focus on the future, and making things better.
• Because I'm trying out a new policy of saying basically every single thing I want to say. It's nice, in a lot of ways. Maybe not nice, in others.

Because we need you to understand.

I have two mental illnesses and it doesn't bother me a bit for anyone to know. I think it is that I don't identify myself with those conditions, I don't define myself in those terms. Because of that, it just doesn't come up very often. When it does I am happy to talk about it; it is like describing some trip I have taken, a place I have been.

I think that maybe that attitude is more common nowadays, but I still come across many people who carry their illness like a secret cross they bear, and when they do talk about it is with a victim mentality that is hard for me to take.