"All the world is my canvas, and I must paint it!"
Everything the Artist sees inspires him to paint. The sunset, the sky, the bugs, passing airplanes...each one gives him the idea for a truly magnificent work of art, and he is consumed with painting them until he is done.
Since he doesn't carry a canvas with him everywhere he goes, though, this leads to him painting on whatever surface happens to be handy. He paints on walls, rocks, trees and anything else he can find. You'd think it might get him in trouble, but his artistic creations are so spectacular that no one ever seems to mind!