@Dutchess_III is right, it also has to do with not having regular touching and skin to skin contact. Try running your nails lightly up and down the inside your arm its quite a good feeling for the same reason, the other side gets touched all the time so its not as sensive.
The act of scratching your skin (the way you would to relieve an itch- not like when you get hurt) produces a brain reaction that is pleasing for a couple of reasons. It reduces activities in the brain such as thinking of difficult or painful experiences while increasing activity in the brain linked to touch, pleasure and obsessive behaviors….
My old, old gramma used to do that @Schroedes13. She was from the old country and she would drink her “red medicine” (some sort of liquor) and rub her back on a door frame and sing songs in Dutch and her her support stockings sagged down around her ankles. Do you drink your red medicine and sing songs in Dutch when you’re rubbing on the door frame @Schroedes13? Heh! Pokin’ the new guy! Glad to see you still around!
I would guess it’s because it’s not so easy to do, like scratching your thigh or your shoulder. It is definitely one of the most awesome feelings in the world, though. I would guess that anyone willing to scratch my back is probably automatically placed in the “friend for life” category.
One reason might be that because the back is less immediately scratched, the itch has longer to build intensity, so when it finally does get scratched, the relief is more intense, and therefore more pleasurable