I'd guess that a significant contributor of it is that it is so socially conflicted. Teenagers tend to go into puberty with huge amounts of conflicting information from peers, the media, parents, and whatever else and as such develop their tastes in highly unpredictable circumstances. After that, it gets a bit simpler, but only a bit - sex is still damn contentious but not openly spoken about in many circles.
Combined with the basic drive for sex (which means that nearly everyone has a desire to participate in it) with the highly unpredictable yet nominally repressed social view of it, you get a smorgasbord of of potential associations that people pick up on, and base their fetishes off of mostly subconsciously.
Combined with the basic drive for sex (which means that nearly everyone has a desire to participate in it) with the highly unpredictable yet nominally repressed social view of it, you get a smorgasbord of of potential associations that people pick up on, and base their fetishes off of mostly subconsciously.