Why do guys like to smack a girls ass during sex?

It's a crime of passion almost. You just get caught up in the moment and you want to do it to see how she's going to react. Sometimes you do it just as an experiment to see what she likes and sometimes it's a male dominance thing.

A lot of girls like it though in my experience as long as it's not too hard. Also, sometimes it's just part of rough sex, and you can't really be too conservative because then there's nothing really rough about it. I've had girls who love it when I pull their hair, slap their ass, even lightly choke them. Sometimes she just wants me to treat like just a piece of meat, she gets off on it, and I don't mind playing to her fantasy. There's always an extreme though. As of yet though, I haven't had to crack out the ball gag and whip for a girl. I've thought about it and although it'd be awkward for me at first I think, if she was really into it I'd indulge her. I'd certainly never suggest it though, haha.

Getting back to ass slapping I imagine she likes the fact that it's kinda taboo, or maybe she's even into the whole male dominance thing as well. Part of it is just the natural contrast between pain and pleasure, it just adds a lot of perspective. Pleasure becomes a lot more pleasurable when directly compared the pain and it's not uncommon for people to actually respond positively when experiencing two extremes at the same time. It's like when you enjoy the taste of something that is both bitter and sweet, or you are soothed by menthol rub that feels both hot and cold. People even blatantly speak about it. When it's really hot, they wish they could just jump into the arctic ocean, when it's really cold all they can think about is spending a day in the desert. No body wants to just find a place that's room temperature because it just wouldn't provide as much relief as the extreme.