What's to elaborate? That your partner's smile seems to be directly connected to endorphins in your bloodstream? That knowing that your smile has the same effect on them makes it twice as potent? That your baby's first laugh or step makes all the sleepless nights to fade away? That your dog happily wagging its tail when you come home cheers you up after a hard day? "The ultimate incentive to cooperate" seems like an apt decision-theoretic description of what evolution wrought.