When someone smiles at us, we are programmed to smile back. This is very evident with infants, but it is also very noticeable with other adults, even between strangers. This is true with all facial expressions. If you frown, there is a strong tendency to bring on frowns in others; angry expressions elicits others to be angry, etc. This is not just a Sesame Street jingle, but scientific fact. Much of this, indeed most of it in the first second or two, occurs below the level of consciousness.