Great question!

For me its not spanking specifically. its the pain and my ass is one of the safest places to hit repeatedly.

I am way more masochist, than sub. I tend to think of myself as a bottom although i am fine incorporating some femdom in a session. I dont have a domestic discipline fetish. I also dont want to think of pain as a punishment. I am turned on by it and enjoy both me and my partner having fun with beating me up.

For what its worth my pain tolerance is pretty high and on my vanilla life i enjoy a lot of hobbies that involve some level of pain. Hard workouts, ultra running etc. i think it us just how my brain is wired.