Actually having a fandom is a big part of what will get me to write fic - it's especially influential in getting me to write LOTS of fic.

I write original stuff, for publishing, and with that I have to be able to write it and know I'm not going to get much feedback, if any, and not right away. Fic in fandom is part of a conversation, so if I write something and no one responds, or there isn't much response... it's unlikely I'll keep pounding out fics.