Originally I posted this as part of an answer in a thread on a writer's forum:
I write because I think I have some interesting stories to share.
I write because it makes me a better English teacher.
I write because it allows me to earn CEUs to maintain my teaching license.
I write because I enjoy it.
I write because it helps pay the bills.
I write because it's neat to have my name on a cover.
I write because I want the opportunity to meet readers and other authors at signings, events and conventions.
I write because it enables me to share and help others to achieve their writing aspirations.
I write because it keeps my mind active, and I am working--being productive.
I write because it's a good example for my daughters.
I write because it's a challenge and a form of competition.
I wrote my first novel because back in college it was something promised myself I'd do one day.
I write because I think reading is important, and having a few more good stories out there counts for something along that line.
I don't write in hopes of earning massive royalty checks.
I don't write because I crave fame.
I don't write to prove a point to anybody.
I don't write because I have a vast (or even a small) amount of spare time to fill.
I don't write because I enjoy rejection and less than positive reviews from readers.