Just how do people get into these weird fetishes in the first place?

I assume you are talking about a real fetish not just something someone happens to like to do as a preference and call it a fetish. I wrote a story about that, it is pretty complicated and starts very early in life.

https://similarworlds.com/2449251-I-Am-Facinated-By-Human-Behavior/428934-What-causes-all-these-fetishes-It-makes-no-sense