I was saddened to hear of the passing of Robin Williams. My heart goes out to his family during this difficult time. It's so unknowable why the ability to bring the gift of laughter to the world frequently goes along with deep mental anguish, but it too often does.
Most of us have had our lives touched by the kind of depression that leads a person to want to end their lives, either by personal experience or by having it affect someone we love, I know I have. I agree with the thoughtful responses I have read here, especially NorCalZombie's advice to those going through a crisis driving them to consider suicide.
"The only thing that will redeem mankind is cooperation." Bertrand Russell - printed on a beer mat in "Shaun of The Dead".