As a person who has been a writer for all my life, both published and non-, and who was horrified to discover she had spent several months writing a fanfic (oh, the shame) because the characters wouldn't leave the brain, I agree with Cory's explanation of why people write about other people's characters.

And here's a question: why are Joan Aiken's novels about Jane Austen characters legitimate literary works and fanfic based on TV characters not legitimate literary works? (Assuming that they're well written.) I have the same biases, by the way, even though I wrote a Firefly fanfic -- it's somehow a little shameful to admit spending time in such a pursuit.