Manners are a product of civilization.
Widespread rudeness is the result of a civilization in decline. A person should be polite, although not for the sake of others. Living for the sake of strangers leads, quite appropriately, to resentment and, not surprisingly, rudeness and hostility. A pride in oneself and a desire to live in a civilized, decent and benevolent universe ought to be a motivation for manners. Look around at how people dress and act, and you’ll get a clue as to how they think and feel. Manners are not the only indicator, but are nonetheless a very good barometer for where society and civilization are headed.