Why do some people do this?

Why do some couples consent to monogamous relationships, or prefer the "swinging" lifestyle? For people who have actually done it, I've just wondered what exactly about seeing your partner with someone else gets you off? I don't completely understand the concept because I believe in faithful, one-mate relationships and my jealousy would prevent me from participating in cohabitation. So for those of you have done so before, or believe in it, explain the experience and give reasons as to WHY.