Back in college, I took classes on criminal profiling. All sexual behaviors, abnormal and normal, have the following four components: fantasy, symbolism, ritualism, and compulsion. Naturally enough, the key difference is when the need to commit an act overtakes control of someone's life. Fetishes arise out of the feedback cycle in which there is greater desensitivity, less reward, less pleasure, as they commit the acts, and thus they gradually up the ante to maintain the high. Sexual offenders in particular usually start off innocently enough as peeping toms and exhibitionists, then progress into bizarre fetishes. They characteristically have multiple fetishes.

The origins of fetishes are still largely unknown, as there is nothing these people which in any way is unique to them, although there are an exceptionally high rates of alcoholism and disruptive/abusive childhoods, yes. Pent up sexual deprivation is also an usual suspect; they have a sort of "intimacy" with the object because they can't achieve intimacy with human beings.

Strictly speaking, there is no such thing as a feet fetish, that's a partialism. Everyone has partialisms. The word "fetish" is thrown around so loosely nowadays that it has lost much of its clinical significance, but it's important to understand its classical definition when trying to understand why people do the really bizarre things such as "defecate on your chest or throwing up on your nuts."