One of the little quirks of my family - which goes back to when I was a kid and probably goes back generations - is the enjoyment we derive from having our back scratched. Not rubbed - but a light nail scratch gently across the back.
In my personal experience, the "rub" is greatly inferior to the "scratch" in terms of pleasure, the rub feels good, but the scratch clearly does some alteration to the entire body and brain while it is in process. Relaxation, almost to the point of falling asleep, is the result.
I've often wondered, and still do, why a slightly sharper but smaller point of contact on the back is so much more impacting than a larger, duller contact - the different between fingernails and fingers. Perhaps science is starting to unravel this mystery.