This is a question I have often asked myself, in regard to a number of things, but particularly writing. And having looked into this a little I have found some interesting theories and surprising reasons why people write, as well as some really inspirational quotes on writing in general.
Here are a few sites I found interesting:
Why Do Writers Write?
Why Do Writers Write? NYTimes.com
Mark Coker: Why Do Writers Write?
And my favourite, from my daughter:
"When Daddy sells his book we can go on another holiday."
And here are are some quotes I found inspiring or instructional, or both:
"Keep away from people who try to belittle your ambitions. Small people always do that, but the really great make you feel that you, too, can become great." Mark Twain
Richard North Patterson
Most of these and many other great quotes can be found here: