Comment by hackit2

In high social cohesion there is social pressure to adhere to reciprocation, how-ever this start breaking down above a certain human group size. Empathy like all emotions require effort and cognitive load, and without things being mutual you will eventually slowly become drained, bitter and resentful because of empathy fatigue. To prevent emotional exhaustion and conserve energy, a person's empathy is like a sliding scale that is constantly adjusted based on the closeness of their relationship with others.