The question of why Charles Burchfield painted in watercolor comes up often. There's a great quote in the Archives of American Art interview with John D. Morse. More at http://www.aaa.si.edu/collections/interviews/oral-history-interview-charles-burchfield-12702
"why did I like watercolor over any other medium... because I've used it ever since I was a child, and used it -- it never presented any problems. It's as easy for me to work in water colors as for the ordinary person who isn't an artist to use pencil or pen. There's no thought required. The only thought that is required is what I'm going to do, what I'm going to put down. But the putting of it down is just as simple as breathing. And that being the case, I couldn't possibly tell anybody how to paint in watercolor. And that's what they wanted to know. And lots of times I was asked to teach, and students would enter my class hoping they would find out something about how to handle watercolor, and I would try to tell them. I told them: "Don't think about the medium. What you're trying to say is much more important that what you're saying it with. And if you're thinking about what you are trying to express, you may use watercolor like nobody else ever used it. And that's all right as long as you say what you want to say."