I find it most interesting that many of these interactions are analogues to a petting motion. I don't think it's an accident that we find these petting motions

I find it most interesting that many of these interactions are analogues to a petting motion. I don't think it's an accident that we find these petting motions reassuring either. I would guess that it might have something to do with nit-picking (think early ape-like ancestors), or possibly something to do with bonding with dogs (who use rubbing motions of these sort to determine pack order). Probably the former, since, while man's interaction with dog is very old, it's probably not old enough to have evolved a self-gratifying feedback loop.