The Wisdom in Manners…
Answer by Yishan Wong:
My father explained it to me thusly, that “politeness is the social lubricant between strangers.”
Between friends, politeness and manners may not be necessary, because you are aware of each others’ propensities and individual sensitivities. However, this is not the case when interacting with strangers. Thus, manners are a basic set of interaction protocols between members of a common civilization, to avoid the chance of giving unintended offense to a stranger whose particular sensitivities are unknown, and to convey your good faith.