In my case, I feel it could be due to not seeing everyone’s feet all the time. Just like you don’t normally see their chest, or their dick or ass, feet are normally covered, so that turns me on a lot, bc it makes me feel like I’m seeing some “secret” part of their body.
to piggyback on that idea, there's a certain level of intimacy between people that exists before you are barefoot around them casually. You don't normally just kick off your shoes in a stranger or mild acquaintance's house. It's somewhat symbolic of some level of deeper relationship, not even necessarily sexual in nature, more a comfort level thing. There is something to the idea that it's a sight of a person not seen by everyone.
It's been mentioned a few times too that for some, there's a domination/BDSM thing to it too. For me it's not as much domination, but I do have a thing for bondage and usually ankles and wrists are focal points in that practice, so theres some of that in there for me too I guess. I don't know which fetish I developed first but it probably led in part to the later one developing too.
Body language, particularly in response to strong stimuli/pleasure, is something I find hot. Second only two facial expression, feet are pretty expressive of what people are feeling. For example, the toe-curling orgasm is a real thing.
Biology probably plays a role too, as mentioned already there's likely some crosslinking in areas of the brain which add to it for some people, also there are more nerves in the feet than in all other similarly non-sexual parts of the body. Its why people who are ticklish tend most to be ticklish on the feet
People's feet are also visually unique, sort of like a finger print. You can sort of ID people by uniquenesses in their appearances.
