I've always thought of fan fiction as the warped respective perspectives of those who enjoy the series', or day dreams as some have said. I imagine someone thinking about their favorite show (let's just make it Naruto for evil's sake) and they were thinking, man, I think it would be cool if Naruto met the 4th. Then they proceed to write this thought which is probably super exciting for them, onto paper or computer, full of dreamed-up complex, niche action scenes, corny dialouge, and predictable plot progression that is clearly catering to this person's fantasies, and share what they believe to be cool with the rest of the world. There is a high chance that their story is so niche, that only they can like it, but there is also a chance that others will like it as well. I think of it the same way I think of a lot of art, particularly abstract art. It's what it is to the eyes of the beholder. No matter how many people like or dislike something, there will always be someone who will say "this is utter shit" or "this is so heavenly, I just nerdgasm'd". I personally dislike abstract art because I simply don't care about whatever message it's trying to convey (and sell for 2k), and I don't really care. Sometimes, and many times, people will make art simply for themselves to marvel at and enjoy. I generally don't always find beauty in abstract symbolic art like others do, but like fan fiction, I'm cool with it because other people like it, and just because I don't like it, it doesn't mean it isn't liked.