This is common knowledge. Another usual explanation is that the brain produces endorphins, natural opiates, to compensate for pain. You actually get high off the sensation. The “runner’s high” comes from pushing the body painfully for so long that the endorphins kick in; the rush you get after eating chili peppers comes from the same source; and that’s what makes it enjoyable for SM players to be whipped or spanked or whatever. It’s not pain, its pleasure! All athletes that are “hooked on exercise” are essentially masochists who enjoy stressing their bodies to get that chemical response. So your friend who enjoys being spanked may actually be a lot less masochistic than your average marathon runner!
Endorphins are by no means The Single Explanation for why masochists find intense sensation to be desirable. Not every masochist floats away blissfully while being whipped, nor would they all even want to. The ways to experience intense sensation varies from dreamlike rush to stinging ouch to irritating maddening burn to soothing warmth to tears-in-the-eyes throbbing… and whatever the sensation, there is likely someone who enjoys it.
Some people consider all this absurd. “How could you WANT pain?” The best answer I can give is that some people simply seem to be calibrated differently. They want more sensation; they find the intensity thrilling and exciting, whereas someone else might find it overpowering and agonizing. People like different amounts of spice in their food; why not in their sexual encounters? Each person experiences sensation differently, and if you want more, there are safe ways for you to get it. Getting what you want, safely, can make your life much happier.
If you have questions, please feel free to email me. “Education’s purpose is to replace an empty mind with an open one.”- Malcolm S. Forbes.