Do you ever just find your self writing?

I do, about five years ago when I was in High School I just started writing, not for any specific reason but just because I had an idea in my head and I ran with it. Today of that same book that I started that day in high school I arrived at page 400 and I never really paid attention to how many pages I had. I'd write for a little then go back and edit and then continue. Today was the first day that I noticed how long this story was getting and how much farther I probably have to go. I've started other books along the way but this has always been my main project. I don't know whether I'm going to try and publish it or not, I view it more of a personal accomplishment. It doesn't really matter to me what other people think of it, being that no one has read it, the point is I like it. Its a fiction book, I was just wondering if anyone else just wrote for the hell of it and what do you most often find yourself writing about?

I personally dance around on fiction and fantasy.

Jam it back in, in the dark.