>>3771905The best explanation I can give is feet are a very sensitive and vulnerable part of your body you wouldn't expose to someone unless you trust them and they're very often controlled subconsciously so they'll express a lot about someone's emotional state. Like watching someone play with their shoes drives me nuts and comes off as them being relaxed and comfortable around me. Feet are also something you tense up when you're uncomfortable or in pain. So someone with sadistic tendencies would find pleasure in forcing you to reveal that discomfort. That loss of control in holding back your emotions can be hot.
This might be a little off point because I think you can make the same case for breasts and vaginas (or any most other body parts for that matter) but neither have ever interested me. It's impossible for me to reason out why I decided to like feet and penis instead.