People who haven't had to deal with that kind of situation don't really get it. They've only ever had ordinary depression, "the blues", situational feelings that everyone has to deal with because you can't be 100% happy every day all the time.
So, it's easy for them to say, "Hey, it's all in your head! You just have to decide to be happy!"
Except that it doesn't work that way.
The comic strip that uses a mangled hand as a metaphor for depression is apt; the same person who will tell a person with clinical depression "you have to decide to be happy" would never consider telling someone with a compound leg fracture just to "walk it off". The difference is that one is an obvious injury; the other is not.
I know there are people who think my own bout of clinical depression in 2011 was just me being lazy, but those people aren't even wrong. Depression is something I've struggled with my entire life, starting in high school, and what looks like "laziness" to these people has always been a combination of paralyzing anxiety and depression over being unable to do anything about always being afraid.
One of the side effects of taking Paxil has been that my temper has moderated considerably. I used to have a hair-trigger, and almost no tolerance for frustration; since I began taking anti-anxiety medication, though, I've been a lot calmer. And that's so because I'm no longer in a constant state of high alert, needing only one push to go over the edge into full-blown panic. Of course I still get angry and frustrated and annoyed, but because there's not this constant background noise of anxiety, it takes longer for the adrenaline to start flowing and I stay calm longer.
...and when you are constantly on high alert, you get depressed, because when the entire world is a source of anxiety for you it can't be otherwise. Particularly when your anxiety keeps you from doing anything.
I'm still trying to figure out what happened in 2011. I believe I have a pretty good picture of what and why and how; I've advanced far enough from where I was then that I've gotten married and have a job, and I'm trying to do the right things. But--again--there are people around me who don't understand my situation and won't listen to what I tell them about it...and wouldn't believe me if they did.
How much worse would it be for someone whose net worth is in the hundred millions and whose only problems are ones that cannot be solved with mere money? Robin Williams was a very rich man, and my mind can only flail at what sort of demons must have tormented him for him to take his own life, particularly when he was (or should have been) surrounded by people who could tell he was having a severe depressive episode and needed help, regardless of whether or not he wanted it.
Ultimately you can't help someone who doesn't want help, and people who have a history of substance abuse tend not to garner much sympathy when they've just told you off for the umpteenth time that they don't have a problem; you have a problem, so STFU and go away.
I don't know anything about Robin Williams and his situation, but I do know that our society needs to get better at ho it treats mental illness.