The psychology can be complicated. Physically, can trigger the release of endorphins and oxytocin, giving the spankee a "high" and a sense of well-being. Several massage techniques involve tapping or slapping (see http://en.wikipedia.org/wiki/Tapotement), so you could view consensual spanking as a development of that.
Psychologically, it can be a dominance/submission thing. Some enjoy losing control (in a safe environment). Some use spanking for discipline, as a "motivation" for doing boring but necessary stuff such as getting up in the morning. Some enjoy the thrill of being "naughty" and break taboos.
Anyway, if someone asks you to spank them, it's not violence/abuse if you do it. You're doing them a favour. Feeling uncomfortable about it is unnecessary, but I understand that you can't just decide not to feel that way. :p
Because they know that they are going to be bad and want to pay forward for their indiscretions?
It's either a pain/pleasure thing or a dominance thing. Not something I'm into.
You are obviously not the dominant type. Or do not understand the line between pleasure and pain. Fetishes are an odd thing. Very "normal" things can be considered fetishes. Maybe this might help. Deep tissue massage. Hurts, but feels soooo good. Same concept. In a very different atmosphere. Some people, women and men alike, not only want the pleasure/pain but, the dominance of the person in control. Weird to me too. I'm a control freak. So, that doesn't work for me.
Some boys too x)
Im not :P
Why do some guys like feet
its just a fetish bruh