I think some people become more quickly decensitized to practicing the same sexual activities over and over. It's the same thing with porn, they no longer get aroused by the same old imagery so they have to keep bringing it to further extremes to rekindle the excitement.

But of course some fetishes are inborn early on in ones sexuality, and I guess there's no clear answer. One theory on foot fetishes is that the main nerves running from the feet to the brain go nearer to the gentials and thereby relate foot and sexual stimulation in the same areas of the brain. I think peripheral neurology plays a part in a lot of it, even the most extreme cases.