I think it's one of the things that has kept me so in love with writing—no one makes me do it. I choose it each day.
It's also why there are so many people who talk about writing a book without ever actually doing so. Because sometimes the choice is hard. No one will make you get out of bed early to write before you head off to school. No one will force you to edit that scene again because you just know it could be better.
That's why communities like Go Teen Writers, great writing friends, and other writing groups are so valuable. They're the rare place that you can be motivated by others who are working toward a similar end.
What helps you stay motivated?