Through manners one learns that other people deserve respect. That the things
they do for you are done by an act of kindness (ie. gratitude) and not
via an act of servitude (ie self-entitlement/brat). This is for direct
manners dealing directly with people.
even indirect manners. If a child decides to pick his/her nose in
public (of course this has to be trained away slowly...). It is a
statement that I can do what I want and it does not matter if other
people find this act distasteful. Once again the child gets a sense
that they are important and others are not. Therefore they lose the
value of other people via respect. They take others for granted (ie.
lack of gratitude).
a a lack of gratitude can harm one from experiencing the joy of life.
Thus the child is harmed. Thus the child needs to learn manners.
Okay.. this might not survive a philosophy exam.. but you get the point?