Put it in the context of dominance and submission among apes and it becomes clear. The one who does nice things for you is displaying a submissive posture and trying to win your favor. They are probably your social inferior: beneath you in the hierarchy, or at most, a peer. The one who doesn't care about you and does what he wants, shows that he's above you in the hierarchy. And some people just can't resist their internal ape tendency to think "wow that's sure an impressive alpha ape right there." But some, who consider themselves alphas themselves, will take it as a challenge. Etc. etc. etc. All somewhat dreary thoughts in the light of soaring techno-optimist talk about the glory of the human spirit, but I think we're all still apes and I've said so, enough times on HN, that I'm at risk of becoming known as the "ape guy."

Manners are in fact detailed prescriptive means & methods for being submissive, or rather for sending submissive signals, as the default behavior in societies where there are more humans living together than normal, i.e. anytime after 1800 when the population really started to shoot upward and to urbanize. It ends up being very practical to train people to be submissive in an urban industrial society, because if you have a thousand supposed self-declared alpha-apes constantly fighting it out "out there," things become a mess very quickly.

Edit: to the respondents, I am intentionally keeping morality out of it (decency and so forth), because a strictly moralistic right/wrong judgment-based view didn't seem to be enhancing the parent's understanding. But dominance & submission, like morality, is just another narrow rubric for viewing the world, doesn't describe the whole world, and isn't the only way of describing the world. So it's best not to take it too far beyond a blurry big-picture view!