I have very elaborate stories that I imagine in my head. I imagine stuff from the points of view of several characters. I do the same scenes over and over again, think through entire conversations that can last, like, an hour. I think of the back-stories for all/most of the characters, think through random scenes from their pasts. It's like having a novel in there, only, with more information about a wider variety of stuff than a novel.
I'm not sure why I do this, but I always have done it.
It's just this massive part of my life that very few other people seem to experience. Most people don't even understand what I'm talking about: "so, it's like, a day-dream?" - no! The only people that do understand are the few people that do the same thing...