[JenB]

Because I have thought the story through already, and I get bored with writing it.

The one I'm currently writing will, I hope, be different, because this story matters to me. I want to get it out, and get it right, rather than just telling it for myself.

[Fedora]

Makes sense. Yeah, I find that does make a big difference whether you're writing for yourself or whether you know someone else is going to read it.