I suppose it's a bit of everything, personally whenever I start writing there's the initial burst, usually when I have a particular idea or scene that pops up fully formed. After I write about a page or so rapid fire style, I end up deciding that I either a) hate any and everything I write, b) think it's similar to something I've once read, or c) [rarely] decide it's not completely terrible and keep working on it.
That process kinda explains why I have dozens of half written fics on my hard drive that may or may not see the light of day *facepalm*

I'm trying to be less critical of myself, I think that's my worst enemy..