The social custom of reciprocating a favorable activity with another positive action is known as reciprocity. It’s the reason you feel owe someone when they do you a favor.
Robert Cialdini’s book “Influence: The Psychology of Persuasion,” which became well known, The adage “You’ve got to give to get” can be used to describe reciprocity. Customers were 42% more likely to make a purchase in one of Cialdini’s tests, for instance, if they received a free piece of chocolate upon entering the business.