In a Glimmer Train Bulletin, author Roxana Robinson says:
"All the fiction I write arises from the same sort of impulse: it's a feeling of discomfort, a kind of unspecified anxiety, a need to uncover something that troubles and disturbs me. I write toward that feeling."
I'm not sure if she's saying that that is why she writes, but needing to write because you're disturbed and uncomfortable about something would certainly be a good reason as far as I'm concerned.
In this essay, Robinson also gives a great explanation of the difference between writing short stories and novels. And she does it in a nice, concise manner, too.