As Steven Pinker observes, the logic of loyalty is particularly clear in the domain of romantic relationships: You’re a great catch, but there is bound to be someone out there who’s got everything you’ve got plus a little more. Knowing that your partner might someday meet such a person, you’d be reassured by the knowledge that your partner isn’t going to leave you as soon as something better comes along. This would make you much more willing to settle down with your partner and start a family–a high-stakes cooperative endeavor if ever there was one. It’s wonderful that your partner fully appreciates your many marketable qualities, but that may not be enough to keep you together. What you really want is for your partner to have a deep, unshakable desire to be with you and you alone. In short, you want your partner to love you, to want you not only for your wonderful qualities but just because you’re you. Only love provides the kind of loyalty you need in order to take the parenting plunge. Thus, love appears to be more than just an intense form of caring. It’s a highly specialized piece of psychological machinery, an emotional straitjacket that enables cooperative parenting by assuring our parenting partners that they won’t be abandoned.
~Joshua Greene, Moral Tribes: Emotion, Reason, and the Gap Between Us and Them (2013), p. 42