This isn't exactly going to be an elaborate post, but perhaps some of you will enjoy, if you missed it, this article that was recently in Salon.com. It's ostensbily about writers who, after graduating from MFA writing progams, stop writing and start freaking out because they've stopped writing. But it's really for anyone who is questioning WHY they write.
I think this is always an excellent question to ask yourself, and I think it's a good idea to ask it on a regular basis. It helps to know why you have chosen to follow this writerly path, even if all you know is "I have no choice; I can't NOT do it."
But there are also many other reasons to choose from, and you can sort through them at your leisure, accepting and rejecting . . . and also changing your mind again and again, as time goes by and you evolve as a person and therefore as a writer.