I think it may have something to do with the intention. At least this has been my experience. I mean, pain coming from someone you love as a gesture of that love or of the shared passion between you (whether that pain comes from something as intense as a flogging or similar or is just fingernails in the back...mmmm sorry, drifted off for a moment) is entirely different from the random pains that come from day-to-day mishaps or are inflicted maliciously.

That isnt perhaps what you meant by a 'concrete' difference, but it's about as close as I could make it unless you care to give me a research staff, laboratory and equipment. And if you do, I'll gladly research it exhaustively.