Well, the best I can come up with is to tell you why I do it. But there are as many reasons as there are writers -- also there is no WRONG reason to write.
I write because I enjoy it. When it goes right, nothing can give you greater pleasure. I have come to feel about my stories (especially since I started doing longer works) that the true satisfaction is in the work itself. At first, I wanted to sell, sell, sell, and I did publish a lot. Now, I'm more in tune to making the writing as good as I can. The work is my reward, because I enjoy every minute, even the ones when I'm struggling hard. Then if it sells -- great. If it doesn't, I've had my fun. My friend, mystery writer Helen Chappell, says I should be shot for even thinking such a thing and "nobody but a fool every wrote, except for money." But I can't help how I feel.
Yes, writing is hard. Some days you feel as if you're wrestling a bear. But, oh the sense of accomplishment when you make that bear dance!