I agree that probably the biggest reasons are what Graeme said. Having started so many unfinished stories (mostly on other sites) makes me determined not to do that. I either have the story totally done (or within a couple chapters) before posting at all. Lately, though, I've started a story and just made sure I'm 3-4 chapters ahead of what I'm posting, so that I can still post each week, even if I have a week where I didn't get anything written. Knowing I have a story out there posted, waiting to be finished, has actually been pretty motivating for me to make sure I take time for me each week to do what I love to do.