Very simply I write because I love it. I love to write about things that I like, because it makes me feel closer to them. Writing is also a brief respite from reality - I prefer to get lost in a world of my own creation (or somebody else's, if I'm reading) than to live in the media-dominated real world. I suppose I prefer fantasy to fact.
I also write because I have been inspired by books. There have been books I've read that wouldn't let me put them down until I'd found out the ending, or had made me feel things like anger or sadness, even cry! I always think how amazing it would be if something I wrote could have that powerful an effect on somebody else.
I write because it's something worthwhile to be proud of, and when I was younger I always dreamed about having my own books published one day. I doubt it will happen, but one can always dream. =)