What was your earliest remembrance of being spanked by a parent? Had you done something "bad"? You probably didn't think so at the time but when you felt that sharp pain of having your bottom slapped, it triggered a pleasurable response -- the opposite of what your parent wanted you to "learn"!
As you were still pre-teen, how often did you do something "bad" just so you would get spanked? You were manipulating the parental units to give you the pleasure you desired. The hard strike on your butt got your "desire" revved up -- you couldn't wait to go someplace, strip naked and stroke yourself to release!
Part of the desire to be spanked is the sexual result of the action of being dominated by an "authority" and partly the sensation of pain on the ass flesh which can radiate thru the entire pelvic region, stimulating both the penis/vagina and the anus.
Which is more stimulating to you? Asking some to spank you, undressing to expose your bare bottom and accepting their method of striking OR being grabbed by the arm, forced over the lap or held down on a table, feeling your pants and underwear being yanked down your legs forcibly and having bare ass spanked/whipped/paddled until your whole body in on fire, from your shoulders to your ankles?