The pleasure pain is definitely part of it, but also a complete acceptance of your partner. Obviously it isn't for everyone because it is taboo. But it has an overwhelming feeling of completeness to it and deep love as well.
Something about the combination of pleasure and pain at the same time. It gets blood rushing to the right places. A little taboo, a little fun.
Partly it's the pain - pain releases endorphins into the blood which make you feel good, and are actually addictive. But it's also a power exchange and an act of trust for the submissive.
My wife can actually have an ****** while being spanked the right way. I know that she really gets horny when I control her actions and her words while I tie her up and spank her. I have had to learn how to do this safely though since it can be dangerous when done improperly. It's bascially foreplay for some rather rough sex. I have to love it, since my wife loves it and it makes her happy.
It is the feeling of submission.
You are right it is not because it hurts although to be a proper spanking the sting should be significant. It is the submission to my partner and obedience of her commands. There is a fine line between an erotic sting and a nasty biting punishment sting. When the strapping is over I go to the bathroom and check my bum for marks. I want to see some stripes.
I'm a single mom always having to be in control of everything. There is an absolute pleasure, relief and release that comes with submitting to my bf for a good spanking or just manhandling as part of our intimate relationship. He has a very laid back personality so I like being able to see and feel a more authoritative side from him. Outside the bedroom he treats me as an equal, and a princess. What we do behind closed doors is separate and special & keeps us balanced. So yes, it's definitely much more than the pain. It's also trust. I know he won't hurt me or go too far. He knows just what I need from him. :-)