I'm always fascinated by what makes someone fannish about a source. I've seen it argued that flawed sources are more likely to have fandoms (as fans feel they need to 'fix' things), and I definitely think it's a factor, but I believe other elements like existing fannish infrastructure, whether the subject/a character pushes our specific buttons, and of course whether our friends are in it all play a part.

There's something about HP fandom, isn't there? Just when you think you're out, they pull you back in! :D