[W]e live in two worlds: one characterized by social exchanges and the other characterized by market exchanges. And we apply different norms to these two kinds of relationships. Moreover, introducing market norms into social exchanges…violates the social norms and hurts the relationships. Once this type of mistake has been committed, recovering a social relationship is difficult. Once you’ve offered to pay for the delightful Thanksgiving dinner, your mother-in-law will remember the incident for years to come.

Dan Ariely, Predictably Irrational. (ISBN: 9780061353246) p.84