This is an important question. Unless your writing is done solely for your eyes--and there's nothing wrong with that--feedback from eyes that do not belong to you is highly important. For me, in fact, it's a huge motivator. I can't write just for myself (at least, nothing substantial). I've attempted to keep journals and about three entries in, every time, they fizzle out. However, I can write a letter with great enthusiasm. What does that say, that I can write pages for someone else but can't write the same amount in a personal journal?
Well, it says I write to express myself.
If I am expressing myself, then, I want to share. And therefore I need a group of people to share with, people I don't mind reading my rough-drafty sentences when they're hot off the press (er, keyboard) and I just can't wait to share because I'm so, so excited. Really, if there was no one to send it to, my fingers might slow as I reach the end of a scene. Maybe they'd slow as I start. The anticipation of sharing what I',m about to create drives the actual creating. And the feedback received after sharing makes me want to write more. It's the ciiircle of life...oh, wait, that's the Lion King. It's the circle of writing, I mean.
What drives you to write?