It's both interesting and timely that you should ask this question at this particular point in time, as I have the third chapter of My Personal Trainer written and awaiting edit for publication, expecting it will be done soon. Anyway, my alter ego main character in that one kind of asks the same general question of herself (why am I getting off on this?) although that's maybe implied rather than clearly stated. She then uses her kink to ride through the pain, turning something bad into something good.
In this case it's maybe guilt at having lived such an easy and self-centered lacking empathy life, the character in question getting strung up and whipped with her own belt, for the raw amusement and retribution of others, hundreds of miles away. Those other watching people are manipulating things to their advantage with no apparent risk, and they pretty much suffer from the same general affliction as the main character, but they just don't know it.
Anyway, they're my fictional characters and I can give them whatever traits I'd like.
In the real world if the mood is right, and it's done playfully, a little slap on the bottom is like additional foreplay for me. It's also something that could crash the mood cold if my planets aren't in perfect alignment, so as far as a risk vs reward kind of thing; it doesn't happen all that often. Straight up inflicting pain on somebody doesn't do a thing for me personally, and having it done to me likely wouldn't do all that much either. Now, with that being said, there are some that claim that the pain and pleasure centers are near each other in the human brain, and some signal cross contamination can occur under the right high energy circumstances. I don't know if I've personally experienced this or not, (riding that wave of pain filled euphoria) but next day rope burns, bruises and scratches that didn't hurt at all while actually getting them does happen, at least for me. I suppose drugs could do this too, but I don't play with those.
Now from a straight up, never want to experience it for a second in the real-world kind of thing; spread eagle strung up stripped and whipped, up on a platform where a thousand medieval spectators are cheering on your hooded torturer, all while he lashes your helpless naked body time and again with his big nasty whip, all as you wail out in anguished pitiful gasping breaths until you black out... That is the thing of dark erotic fantasy for me.
And to then be revived by several buckets of cold river water being thrown on you, so the hooded, winded, sweating in the hot sun fiend can pick up right where he left off, ensuring that the simple peasants who turned out to see "justice" applied got the full show they expected; simply fantasy nirvana to me, but I digress...
Not that it's a direct answer to your question, but atonement for past sins comes to mind. Another idea that might or might not work for you could be your character asking herself this very question, as in; "why am I drawn towards this, why do I let rough men do what they will with me?" You could write a whole story around her trying to work that out in her head. Is her present husband soft, caring, and loving, and does the hunger for rough things suggest her desire to try something new for herself as a contrast to what is safe and "normal?" I've often thought about this myself, if my husband was an offish brute, I'd likely dream of knights in shining armor, but since he's not, I fantasize about selfish brutes taking physically what they want.
Had your character fantasized about such things since coming of age, perhaps getting her first taste of such concepts at that early informative age, maybe with a borrowed sultry romance novel; these things rewiring her brain to desire this over more traditional romantic things? I've often wondered why certain things excite certain people; something built in, or something conditionally learned someplace?
It's a great question that's made me think a bit, always a good thing, Jackie.