This was sort of a random curiosity that popped into my head when I was browsing around (and sorry if this came up before) but... anyhow!
I gotta say though, we're all some form of masochists. I think you gotta be to be involved in this community.
I don't think there should be a probably in that sentence. It's definitely one or the other unless you're into doing both. Either way even that's a definite choice.
You obviously aren't very familiar with the bdsm community if you haven't heard of switches. This gives a basic overview, particularly of some misconceptions: http://www.submissiveguide.com/encyclopedia/switch/
What do you mean how? You just are, or you're not. And technically with the switch thing, you go from one to the other, rather than be both. Though you can be both too. If you're the type to scratch/bite your partner during sex, you're probably a sadist. But then, maybe sometimes you want to get choked too.. :x I think it depends a lot on mood and your partner; humans are inherently adaptable, after all.