SNM:
Is probably the only answer you're going to get on an adult spanking site. And its a perfectly good answer. Some people like spanking. Some people like being spanked. When they do their thing together, everyone has fun. What more are you looking for?
Well, I'm thinking that this question isn't about a playful/erotic spanking relationship that works out as "everyone has fun."
I think it's more about a serious domestic disciplinary relationship in which an otherwise independent individual, intelligent, fairly responsible and self-aware, submits to being soundly corporally corrected by someone else whom the first person isn't generally inferior to.
How does a person reconcile him/herself to accepting that another person, generally no more intelligent, educated, mature, etc. than he/she is, nonetheless can decide to punitively paddle his/her bare behind (for whatever reason) and that he/she has agreed to cooperate in undergoing such punishment at his/her disciplinarian's discretion?
There may be mutual benefits to such a relationship, one or both of those involved may even crave it, but how does the spankee feel about that loss of personal choice and how does the spanker feel about having that kind of power over another otherwise independent individual? Agreement to such an arrangement doesn't preclude thinking about it analytically, especially when the disciplinarian announces that a spanking is going to be administered that the intended recipient doesn't feel is justified--but has already agreed to accept at the other person's insistence. (This is fairly common in stories about this kind of relationship--the spanker agrees that his/her decision may be "discussed together," but it's still his/her final decision as to whether or not the discipline is meted out.)
My personal spanking activity involves exclusively marital play/fun "punishments," but I still occasionally ponder on what a serious domestic disciplinary relationship with my darling wife would be like--notably when we've argued and/or she's angry with me over something. What if she had the personal authority (that I'd ceded to her) and the inclination to give me a very sound, truly punitive paddling on my bare behind, one that wouldn't stop until I was sobbing and sincerely regretful--even in situations where I didn't feel I was at fault?
Experiencing this kind of situation is unlikely to ever happen to me, but it's something that I deal with in my stories--most of my fictional adult spankees aren't effectively "children" (although they may be mildly treated that way for embarrassment purposes during a disciplinary session) compared to their spankers, so it can be a tricky situation when they're being spanked as behavior-modifying chastisement rather than as play/fun.
That kind of domestic discipline relationship may exist and work effectively, but IMHO it's certainly not always easy to justify rationally... --C.K.