Heh, I don't know! Part of it might be that I basically never even start something unless I'm sure it's something I'm also going to finish. (I might jot down ideas about other potential stories, but I don't start writing actual sentences of a thing unless it's something I know I want to see through to completion.) And then I guess my perfectionism (not always as good a thing as it sounds like...) and obsessive need to keep things tidy and complete carry me through from there. :-P