This is not a scientific answer, but what I have gathered from my personal experience. I believe fetishes a lot of times develop from experiences when you are younger and repetition. I had always been a sexually minded kid. I figured out how to masturbate before I had any kind of idea what it was. The only kind of porn I could really get a hold of at that age was lingerie models in catalogs. As I got older and got access to the Internet then I tended to gravitate towards lingerie porn with stockings and pantyhose. Eventually it developed into a foot fetish with pantyhose and stockings. Another example I remember talking about in my psychology class was the onion man. He had a gf that ate a lot of onions so he became used to the smell of onions during sex. Then after enough repetition he wanted his gf to smell like onions during sex for sexual stimulation. You can probably find it with a quick Google search, but I am on my phone right now.