I'm fairly certain most people agree that what causes the plethora of sexual attractions, fetishes, what have you is a mystery. For example, when Master and I are doing it doggy-style, it drives me crazy whenever he grabs my ankle. I don't know why this is such a major turn-on for me, but nevertheless it is. M, on the other hand, can be reduced to an incoherent puddle by applying pressure to her collar bone - not even biting it, just pressing on it with your finger. I'll bet that everyone has their own little sexual quirks like these, and that no one can really explain why. Most people are pretty understanding of this, too; when confronted with a desire that they can't understand, many people will just shrug and say, "Hey, if that's what you're into."
The biologist in me wants to say it has something to do with genetics, since I've never had any environmental exposure to ankle-grabbing that would lead me to view it in such a positive manner. But far be it from me to be a genetic essentialist who views every aspect of personality as something that can be boiled down to a few lines of DNA. (TATTC causes an ankle-squeezing fetish, while TAATC would cause a propensity for water sports? Probably not.)
After all, sexual desires are far from immutable. Incredibly enough, when I first started dating Master I wasn't turned on by spanking in the slightest. He tried it once or twice during sex, and when he asked me whether I liked it, I believe my exact response was to shrug and say, "Meh." But for some reason I grew to like it. It wasn't because Master pressured me; he stopped it after I told him it wasn't a turn-on, and only once we started using other aspects of BDSM, like bondage, did he reintroduce it. By that point, I had suddenly become a spaking whore. Even when it doesn't turn me on physically (get me wet, that is), I just love the physical sensation of a good spanking or paddling.
Everyone involved in BDSM - and increasingly more people who aren't - know what limits are. When Master and I began our first forays into kink, I found a "limit checklist" designed for people negotiating a scene. It was, in essence, a list of various kinky activities, which one could either mark as "major turn-on," "curious about," "not too interested but willing to try," and "hard limit; never ever ever." I filled it out for myself and showed it to Master, and that was our starting point. As I've pulled up the list since then and revised it, I've noticed an evolution - a softening, if you will - of what I'm willing to do. I still have some rock-hard limits (bestiality, scat play, watersports, anything involving breaking the skin, incest) that are so ingrained into my psyche by biology and society that I doubt I could ever be swayed. But there are others (anal play, sensory deprevation, and a whole host of others) that are now piquing my interest when they haven't before. For some of them it is because Master has introduced them to me and I liked them; for others, something about myself has changed, and even though I've never tried them before I suddenly think they sound interesting and fun.
It's for this reason that I'm skeptical of people who say they were "born" kinky/poly/some other sexual descriptor. I mean, certainly genetics plays a part, but we are not static creatures. We change a hundred times over the course of a lifetime, and though the seeds of kinkiness may have been there since birth, it was a million other influences that shaped me into the exact incarnation I am today.