Parents usually do it subconsciously, little kids love it, I've never met someone who do isn't love someone scratching their back and some people even get sexual stimulation from it. What is up with back scratching? ?
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Humans are social creatures.
Have you ever noticed that when you're at your barber, you just can't get enough of them rubbing their hands all over your head?(just me?)
Same thing with your back, among other places.
In our ancestral environment grooming not only facilitated good hygiene, but was one of the main forms of pro-social activity. It tells you you're safe, in the presence of your friends, and in a situation where you can relax.
Though the hygienic purpose has been displaced, we haven't really changed. It still feels nice to gently touch each other in a non-sexual way. It's still a simple animal comfort and also explains why mammals, such as cats and dogs, enjoy being petted.2