Matt Curran set this particular hare a-running over on his blog, and I plan to dodge it for a moment because I've got a fever and am thinking none-too-straight (otherwise I would never have perpetrated a mixed metaphor such as 'dodging a hare'. Though it's not impossible, I suppose. In fact, the hare would probably happily dodge you.)
For the moment I'll take the lazy way out (cf. the first citation below) and lay down a few quotes from other writers about why they write:
All writers are vain, selfish, and lazy, and at the very bottom of their motives there lies a mystery. – George Orwell
If there's a book you really want to read, but it hasn't been written yet, then you must write it. – Toni Morrison
If I don't write to empty my mind, I go mad. – Lord Byron
First, try to be something else, anything else. –Lorrie Moore
It is impossible to discourage the real writers - they don't give a damn what you say, they're going to write.
– Sinclair Lewis
Writing is a socially acceptable form of schizophrenia.
– E.L. Doctorow
Writing is a way of talking without being interrupted.
– Jules Renard
It’ll get rid of all those mood swings you’ve been having
– Ray Bradbury
...there was nothing else I was made to do. – Stephen King
Because I’m good at it. – Flannery O’Connor
I wish I could say that Flannery's answer summed up my own, but almost any other quote on the list comes closer.