After reading Pamela Druckerman's French Children Don't Throw Food, something dawned on me. Druckerman explains that in most cultures, children are taught two magical words related to manners: simply "please" and "thank-you". In France, however, they are taught four, with the addition of "hello" and "goodbye" as compulsory phrases each child should learn and use. 

By greeting another person, we essentially "recognise" them as a person: another individual with wants and needs that are as important as our own. French children who refuse who greet adults when they visit are apparently considered ill-mannered, whereas in England (or at least, for much of my own experience) adults do not expect children to recognise them in greeting (and often forget to greet the children in return).