Rational people gravitate toward rational explanations. I am guilty of same.
Another aspect we have not explored is the entertainer's lifestyle. All entertainers, be they movie stars or stand up comics, have an enormous amount of down time. They have month after month of unstructured free time, stretching before them, which gives them ample opportunity to become bored and/or overly insightful.
In my personal life, the craziest, most depressed, most self-destructive period ever was a six-month period after graduating from college in the middle of the severe recession of 1981. I literally had no structured life -- and it almost drove me insane.
That's when I learned that our grand parents were right: Idle hands ARE the devil's work. And too much time to ponder life is unhealthy.
Now, if I couldn't take six months of that, how does a guy like Robin Williams cope with decades of it? Obviously, it got to him.
I think it was Harrison Ford who was quoted as saying that being a movie star was incredibly unhealthy, mentally. It's why he threw himself into aviation and kept up his carpentry skills.