But it's still pain! There's nothing fun about that!
It's all about context. In the right context, with the right person, inflicting pain is deliciously erotic.
Even a hardcore, "whip me 'til I'm black and blue for a week" masochist doesn't enjoy stubbing his toe. It's not about pain per se.
Pain, like all sensation (in fact, more than other sensations) is contextual. When you are aroused, things feel different than they do when you're not. This is something I suspect nearly everyone has experienced; for example, having someone lightly stroke your back might feel good in some situations, feel fairly neutral in others, and feel unpleasant in others, depending on a zillion factors ranging from who's doing it to how hot it is in the room to whether or not you've recently had an unfortunate encounter with a large box of thumbtacks and a bed of stinging nettles.
Pain in an erotic or BDSM context doesn't feel like that same pain might feel in a different context. Seen through the lens of sexual arousal, pain can make the arousal more vivid, more brightly colored. It adds spice to the arousal. And I say this as a person who's not even that much of a masochist.
Pain triggers different responses in different people and in different situations. A lot (not necessarily all, but a lot) of pain in a BDSM context is worked up slowly, starting with mild sensation and building gradually to a stronger sensation. Many people, when they are built up in that way, will experience an endorphin rush that causes a high similar to a runner's high. If you walk up to someone who's a masochist and you just punch her, she's not going to get that gradual rise in endorphins, so the pain isn't pleasant at all. On the other hand, if you build her up gradually, starting with light spanks and then working up to more intense spankings, it literally *does not feel* the way a hard spanking with no build-up feels. It's not "painful" in the way that, say, just getting spanked or punched or stubbing a toe would be.
We think of pain as being a specific kind of sensation, but it's not. The brain's interpretation of pain depends enormously on context.
For example, I have a partner who loves the feel of a crop. I generally start by laying the crop gently across my partner's ass, just below the curve where her ass meets her legs, so that she knows where the first blow will land. I leave the crop there for a moment, pressing it against her skin, letting her imagine what it's going to feel like.
Sometimes, I'll start tap-tap-tapping her ass with it, in a gentle stacatto, to let the anticipation build and to start the blood rushing to her skin, sensitizing it. Other times, I'll just wait, without moving, until she relaxes.
Eventually, without warning, I'll lift the crop and bring it down very quickly, so fast that it whistles as it moves through the air. She will usually scream when the crop lands, arching her back and thrusting her hips upward against the crop as it lands. Her body tenses and quivers, and the first welt appears almost instantly. Sometimes, I'll pause for a minute, waiting for the stinging to pass and her body to relax before I bring the crop down again.
Other times, I'll place my hand on the small of her back, holding her against the bench as I keep the crop moving, nonstop, WHACK WHACK WHACK, each stroke leaving a bright red mark behind. When I do this, she will continue to scream, each stroke drawing a louder cry until they all blend together. At that point, I'll stop suddenly, and wait for the endorphins to hit.
It only takes a second. She will relax against the bench with a dreamy smile on her face, and then the laughter comes. She'll laugh uncontrollably, her face flush, totally unaware of everything around her.
That's the kind of build-up I'm talking about. The entire experience--the fact that we have been lovers for a long time, the fact that we do this as a form of foreplay, the way that it builds up, the way I watch how she's responding--is all important to her perception of the sensation.
After the first rush subsides, I'll start with the crop again, tapping her on the ass or the back of her legs, building the intensity very slowly. When she starts to moan under the crop, I'll bring it down hard again, leaving another welt. At this point, she'll usually start laughing instantly. I will keep the crop moving, striking her hard three or four times in quick succession, until she's screaming and laughing at the same time. I can keep her there for quite a while, floating in a place where the world is more or less a peripheral blur.
When I finally finish with the crop, I'll stroke her gently with a soft piece of fur, watching her quiver and moan and sigh. It takes a long time for her to come down; it's usually several minutes bfore she can even stand. She floats there dreamily for the longest time, grinning and totally high. And it's very, very exciting to take her to that place.
In a different setting, the same sensations probably wouldn't be pleasurable to her. In that way, it's like any other form of stimulation; the feel of a lover's kiss, no matter how sensual, might be annoying to someone who's concentrating on a difficult job with a tight deadline. The sound of the London Philharmonic might be stirring when you're out attending a show but unpleasant when you're trying to help a family member with math homework. We can easily understand how the perception of just about any experience we have will change according to the context; pain is no different.
Now, of course, it's different for everyone, and no two people experience the same sensation the same way. Not everyone who's into BDSM likes, or is even interested in, pain. The things I've written here don't necessarily apply to everyone. But for some folks, the experience of pain isn't really painful when it's the right sort of pain with the right sort of buildup from the right person in the right context.