Life has a lot of unwritten rules, like don't eat tuna salad in the elevator and it's not okay to compare birth marks in the locker room. Unlike your parents' copy of The Joy of Sex, you can't sit down and memorize all the unwritten rules, so you have to to pick up on the scorn of strangers.
With a little trial and error, you'll learn that too much eye contact results in restraining orders and not everyone likes to admit they wear artificial hair.