Why does pain turn some people on?

I was talking to a friend of mine recently and he was telling me how he recently met a girl who was into the same things sexually as he was and when I questioned what exactly he meant he said he was really into asphyxiation and waterboarding during sex and he met a girl who also was deeply into this too. I mean I know this isn’t common but what I mean is it just the excitement of mixing pain with pleasure or is there more to it?

I mean it’s definitely not my style at least. lol