" Trading favors, the relationship tit for tat that social scientists call reciprocal altruism, was long thought to be the basic backbone of friendship. But recent research has revealed that we actually care less about 'fairness' with our friends than we do when dealing with strangers and acquaintances. In a friendship, when either person insists on repaying a favor it's seen as signaling a weakness in the relationship. Friendship is what happens beyond the tracking of favors.... Among the traits exclusive to -Homo sapiens-, altruism and selflessness are near the top of what makes us human. "

― , We Need to Hang Out: A Memoir of Making Friends