Mutual cooperation gives you a stake in your partner's welfare, especially if they are irreplaceable.

Why do we care so much for friends-much more than one might predict from reciprocity alone? According to a recent theory, organisms who cooperate with each other come to have a stake in each other's well-being: A good cooperator is worth protecting-even anonymously if necessary-so they can be available to cooperate in the future. Here, we present three experiments showing that reciprocity creates a stake in a partner's well-being, such that people are willing to secretly pay to protect good cooperative partners, if doing so keeps those partners available for future interaction. Participants played five rounds of a cooperative game (Prisoner's Dilemma) and then received an opportunity to help their partner, without the partner ever knowing.

Why Hate the Good Guy? Antisocial Punishment of High Cooperators Is Greater When People Compete To Be Chosen.

When choosing social partners, people prefer good cooperators (all else being equal). Given this preference, people wishing to be chosen can either increase their own cooperation to become more desirable or suppress others' cooperation to make them less desirable. Previous research shows that very cooperative people sometimes get punished ("antisocial punishment") or criticized ("do-gooder derogation") in many cultures.