Robert Cialdini is an emeritus psychologist at Arizona State University who studies how our behavior is affected by social rules that we're only vaguely aware of but which have incredible power over what we do....

"We are obligated to give back to others, the form of behavior that they have first given to us," he says. "Essentially thou shall not take without giving in return."

http://www.wbur.org/npr/165570502/give-and-take-how-the-rule-of-reciprocation-binds-us