Fanfiction seems to center around a devotion to another story that already exists. It’s like having a role model or looking up to someone, only that someone is the world of Harry Potter, The Twilight Saga, or even the Bible. Fanfiction writers get inspiration from the stories they love and almost anyone can do it since many books, movies, T.V. shows, etc. have thousands, if not millions, of fans. I think people who write fanfiction have a desire to be part of something bigger by writing to expand the boundaries. Star Trek fans literally want “to boldly go where no man has gone before”
This cheesy video does a good job explaining the laws of copyright and fair use, but the song might get stuck in your head.