It is interesting, I’ve heard in psych circles that even childhood trauma can influence fetishes. Like, if a woman had an abusive father then she will have a kink for being gang raped by burly men. I think I heard it’s why so many pedophiles were molested as children themselves.
Theres also autism, where it dosent exactly arouse you insofar as it fascinates you. A lot of autists have described their fetishes to me as “It doesn't really arouse me, in so much as it fascinates me.”. I know I sound like Taxxon but I believe what happens is autists will attach to something bizarre that will become their whole lives, to the point where it bleeds into the sexual. So it’s less so “I have a shoe fetish” and more so “Shoes are everything to me, I love them so much I even fap to them.”.
Even with autists, childhood influences this. I knew a lady who watched this scene from this kids show back when she was a lil girl, it took place in this Arabic setting, it stuck with her so hard that well into adulthood she still obsessed over sexy gypsy girls. Furries will describe their fetish as originating from a childhood obsession with animals. Some cite bullying which literally made them feel inhuman.
Theres a lot of elements, but as for your comment of “I wonder what this stranger’s fetishes are”, it really just depends on their life so far. If it’s been completely smooth sailing and cushy then they’re most likely very vanilla. If you know they went through some crazy shit then you can trust they’re into some crazy shit. I have never met an exception to this rule.