Nobody has a perfect, definitive answer. There’s a lot of theories regarding how paraphilias form, but limited data, and no real way to prove anything with the data that is there.

The most common theory is that, at some point, the person gets exposed to whatever their “thing” is at a time when arousal is heightened, and the exposure becomes linked to the arousal. Which is pretty fucking obvious, and not exactly a real answer, imo. It’s probably correct, but it doesn’t provide anything useful.

Allow me to paraphrase a story I read in a book about sexual fantasies, that included fetish.

A kid is often tagging along with mom, as often happens. He’s small at first, and just climbs under dressers and into closets to play while she’s getting ready for whatever. One day, little Jimmy is down under a table while mom is getting ready, but it’s for a party at home, and some of the guests are also getting ready.

So, our young fella is seeing all kinds of ankles and shoes, and feet, and occasional flashes of women’s lower bodies. This is an awakening for Jimmy, and he feels the first rush of interest and arousal. Not necessarily an erection, or even a direct sexual sensation. But there’s a heightened awareness that he’s seeing something important.

Now in the original story, Jimmy just developed a general voyeurism and an enjoyment of feet, shoes, and ankles that was higher than usual. It wasn’t a full on paraphilia, nor was it essential for his sexual interests or arousal. But without some variation of that stimulation, he didn’t enjoy sex as much, and his fantasies were more about seeing feet from hiding than actual sexual activity.

When you have a sudden arousal combined with something that’s not really sexual, but still delivers a hit of brain chemicals then you’ve got the recipe for a fetish. It won’t happen to every person because it does take more than one factor happening to build that connection. But when you start looking at anecdotes of people that do know where and when their fetish started, there’s a very high degree of similarity in their stories.

Foot fetishists often talk about feet in the same way others talk about breasts or legs. But there’s also a segment that crosses over with submissive cravings, and there’s a segment that crosses both of those that’s tied strongly to feet being “dirty”. Not always in the literal sense, but as in taboo because feet are often considered dirty in the literal sense.

Because feet get literally dirty, there’s a rejection of them as part of our sexual selves I’m some people. And a lot of fetish builds around taboos.

Which, assuming all the theories hold water, it’s no wonder that foot fetishists are so dedicated. They’ve got an unusual interest that still manages to not be so unpleasant as to render them pariah.