If it's not because of some bad childhood/adolescent experience - it may just be that you've "outgrown the ordinary" in your need for stimulation down below
Because you can't normally feel anything, so when you get hurt it feels like you're actually having sex for a change.
Because women like to be dominated and conquered, it is part of being more yin than Yang.
it stems from the cave man days when we used to wack women on the head for sex
Some people prefer more intensity in order to get satisfaction. Some prefer spanking and other form of pain. I love to have my nipples squeezed hard during sex. It adds to the intensity of my pleasure. We are all just a little different but it is all good..