Flavorwire has published an article about why authors write. There are many links to source material which are fun to explore.
“The best thing about writing fiction is that moment where the story catches fire and comes to life on the page, and suddenly it all makes sense and you know what it’s about and why you’re doing it and what these people are saying and doing, and you get to feel like both the creator and the audience. Everything is suddenly both obvious and surprising… and it’s magic and wonderful and strange.”--Neil Gaiman
My own reason is much simpler ... There is nothing else I'd rather do.