The more and more authors I see lining up on the anti-fanfiction side of the equation, the more convinced I am that their dislike is founded in a basic level of insecurity about their own talents. Not necessarily lack of skill... I believe GRRM is anti-fanfic, too, and he's a pretty damn good writer. But insecurity needs no rationale... and no matter how good of a writer you are, there are always going to be writers who outshine you.

At the very least, there are always going to be writers you think outshine you.

And I suspect that for a lot of these writers who are anti-fanfic, the thing they hate the most is the idea that the existence of copious fanfic for their stories indicates very clearly that they themselves are genre writers. Again, nothing wrong with that, but you'll note that fanfic in regards to classic literature usually does not exist in nearly the same quantity as fanfic for sci-fi, fantasy, or romance titles. So for any writer who has pretensions of grandeur about her own fic, the fact that people are porning it can probably be pretty damn demoralizing. People are always going to want to write more porn about the Lestats and Jamie Frasers than they will for the Rabbit Angstroms and the Jay Gatsbys of classic lit.

These are just my speculations, but I've thought about this a lot. YMMV.