Art is a piece of the inside of a person for all to see, scoff at, or admire. No matter how cynical or biased towards the mundane you are it cannot be denied that the time and energy and imagination and sometimes sheer baldness that goes into a painting, for instance, makes it a piece of a person.
Artists, the famous ones, tend to be crazy people. All the ones I've heard of anyway were loony in some manner. But I think they'd have to be. They're putting their souls on display like that. Just opening the doors in their head and letting the monsters and the cherubs and the weirdness and the simplicity out to fall as it may onto canvas. And then they showed this mess of their souls, these congealed puddles of psychosis, to other people. Our insides are private, that's why they're inside of us. So of course really excellent painters go crazy. They can puke out the truth of who they are. Then, horror of horrors, they can actually look at their innermost selves. Which would be bad, except then other people can see the shapes and colors of the artist's ghosts as well.
True works of art are a person's soul spread around and scrutinized.