Law of reciprocity
“Cialdini, visiting professor of business, marketing and psychology at Stanford argues that there is a law of reciprocity. Send someone a birthday card and they will send you one back. It is why charities give out buttons and pencils when they’re trying to raise funds, or why someone telling us some juicy gossip will likely draw a nugget of information from us, or why it feels awful to say ‘I love you’ and to be met only with a smile.
Reciprocity is a powerful force for social harmony and for doing business.”