Massage is older than humans. Monkeys will often groom another monkey when it becomes agitated. Nit-picking among primates is not just a matter of hygiene, it's a manner of soothing and bonding. All mammals engage in some sort of stroking of one another. Tigers lick their cubs, rats lick their pups. We are biologically wired to respond to touch, to stroking. Massage has been with us since we humans became human, since before we had language. It comes naturally to us.
We take its benefit for granted. We know, from direct experience, that it feels good.