What keeps you writing?

What is it about the stories we write that keep us going long enough to get to the end?

Recently I'd gone through half a dozen story idea, but only made it to the introduction of the story (if I even got that far) before my interest in it dwindled. When I first started, they all seemed like good story ideas, but I started feeling like I was forcing myself to continue writing. It wasn't until I wrote the latest drabble I'd posted that I realized what really motivates me: it's the emotions. I could feel the familiar drumming of my pulse as delved into the deeply emotional experience of a first outing.

There are other things that seem to keep me interested in a story enough to make it to the end, but this seems to be the biggest motivator for me. I'd love to hear what keeps other authors motivated. :)