So it seems like, "Why do people like that?" or "Why do people jack off to that?", seem to be big questions about a special interest. Well there is a simple explanation for it.
Lets say that we have four people, subject A, subject B, girl A, and girl B. Subject A gets loveual feelings whenever he sees girl B. Certain things he likes about her are her socks and her large breasts. Well, when he sees her, information goes to the brain stimulating parts of the brain (senses, thoughts, etc...). Some of this info will go to the part of the brain that creates loveual feelings. So whenever he sees her, his brain tells him, "Wow... that's a pretty hot girl over there because she has some nice socks and a nice pair of breasts.". Now subject B likes girl A. He likes her because of her interest in BDSM. Now normally that is a very disliked thing to most, but his brain is wired differently than everyone else. So when he picks up the info, the parts of his brain that make loveual feelings will pick that up and tell him, "She's hot because she likes BDSM.". And thus a special interest is born.
Part 2: How to Wire Your Brain to a Certain special interest (Coming Soon)