Etiquette is about all of human social behavior. Behavior is regulated by law when etiquette breaks down or when the stakes are high - violations of life, limb, property and so on. Barring that, etiquette is a little social contract we make that we will restrain some of our more provocative impulses in return for living more or less harmoniously in a community.
Judith MartinShould you happen to notice that another person is extremely tall or overweight, eats too much or declines convivial drinks, has red hair or goes about in a wheelchair, ought to get married or ought not to be pregnant -- see if you can refrain from bringing these astonishing observations to that person's attention.
Judith Martin