During puberty, there is a phase when the typical male brain wires itself to sexual signals in its environment. Normally it's the usual stuff the broader culture considers erotic - natural attractors like genitalia, as well as some "invented" things like high heels and lingerie. Feet are kind of on the edge of being intimate - they are usually covered but occasionally you get to observe them and if you have e.g. a crush on a girl and she hangs out with you in her summer thongs (which you will possibly be fixated on, since it's a new part of her body you don't usually get to see) your brain might establish that connection (and you then reinforce it by furiously jerking off to it). I suppose it's also this association mechanism through which you can get fixated on the more weird stuff, like piss or PVC or whatever. Some of these connections are probably easier to make than others, which would be reflected in the relative prevalence of given fetishes in the population.