[sodacat]

I think it's because fan fiction falls into one of three categories: sexual fetishism, wish fulfilment, and original work. The fetishism is there because human sexuality is a freakish little midgit that's always hiding behind some furnature and saying 'Dude, what if octopussy was a literal name? Huh? Huh?'. You can't understand him, you just have to learn how to live with him and clean out his litter bin from time to time.

The wish fulfilment is basically the same time, except it's the author that's enjoying everything. They're masturbating onto the screen and expecting you to watch. The last kind, original work, is fairly rare since most of the people with the talent to write good fiction are smart enough to take their scenario and just create original characters for it so that they own, and can sell, the entire work.