Submission's easy to explain. Pain's hard. It's not just about giving up control, it's about giving up control and being betrayed. If D/s is a trust fall, SM is a trust fall where you hit the ground. Still thrilling, and with a competent top still safe, but... fuuuck, it hurts.
I've heard people say things like "masochists transform pleasure to pain," or "it's not pain, it's intense sensation." Really? Is that what it's like for you? Maybe it is. But for me, there's a lot of real, no-euphemism pain in the experience. Certain types of pain are straight-up pleasurable: very mild slap 'n tickle, pain during sex, and sometimes pain that's sufficiently severe and extended that I get a little out of my head. The meat of a scene, though, hurts me.
So why? Dunno. I don't think it's any kind of negative or self-destructive impulse; hitting makes me happy! I do get a little high afterwards, but it doesn't happen every time and I don't think it's the primary motivation. Ascribing it to The Patriarchy is too ridiculous for words. Maybe it's just one of those random oddities that people are born with. Like an eleventh toe.
Pain teaches you about yourself. You learn how strong you are--I've been whipped with a fucking chain! I've had a knife held to my throat! I've fucked with my tits and pussy bruised purple!--and how weak. Silly James Bond fantasies about how you'd stand up under torture crumble in the face of real pain. Even "moderate" pain can turn me into a sniveling coward, someone who'll kneel and beg and suck cock to make it stop. Some of that is an act, is deliberate submission; some of it isn't. Ultimately there is no strength against pain, and although he could probably take more than me, if you hit him hard enough, James Bond would cry and suck your cock.
(Okay, well, Moore would. Brosnan would be wetting up his lips before you were done with the warmup. Jury's out on Craig. Connery would die defiant.)
Sometimes, though. Sometimes I'm handcuffed up on a chainlink fence, ass-naked in boots and there are strangers staring at me and a man with a cane and a maniac grin is making me cry. Sometimes I laugh, sometimes I know I'm enjoying it, sometimes I'm horrified and begging for it to stop and yet soaking wet between my legs. No, I can't say "every blow is a surge of pleasure." Every blow hurts. And that's awesome.