To seriously answer the question, it's the one of very few things besides painting and swimming that, when I'm really, really into it, I start to feel almost like I'm burning inside. I've tried other instruments, sports, and hobbies, but these are the only things that really do it. It's like my mind is put on another level or something and my hands and chest heat up, and work in perfect unison with my brain. Normally I'm pretty clutzy and tend to make lots of mistakes at whatever I do, whether it be spilling food on myself when eating, having to scribble out a word I made too illegible, etc., but when I get that feeling, my mistakes greatly reduce themselves, and I can work with and make something out of the ones I do make.
Usually I have to be playing with other musicians for this to happen, or be playing for myself without any concern for what's around me (I usually have to be alone in the house), or with painting I have to be working for a while by myself before I really start clicking. I need the right set of circumstances for it to happen, and itt doesn't always happen even when they are right, but when it does, I feel like I know why I'm me, that I'm doing something important, and that there's no one else who can do what I can the way I do it.
That's why I play.