Having established the evolutionary origins of irrationality, Pinker now explains love:

Why does romantic love leave us bewitched, bothered, and bewildered? Could it be another paradoxical tactic […]? Quite possibly. Offering to spend your life and raise children with someone is the most important promise you’ll ever make, and a promise is most credible when the promiser can’t back out. Here is how the economist Robert Frank has reverse-engineered mad love.

[…]

Somewhere in this world of five billion people there lives the best-looking, richest, smartest, funniest, kindest person who would settle for you. But your dreamboat is a needle in a haystack, and you may die single if you insist on waiting for him or her to show up. […] At some point, it pays to set up house with the best person you’ve found so far.

But that calculation leaves your partner vulnerable. The laws of probability say that someday you will meet a more desirable person. and if you are always going for the best you can get, on that day you will dump your partner. But your partner has invested money, time, childrearing, and forgone opportunities in the relationship. [… T]he partner would have been foolish to enter the relationship.

Frank compares the marriage market with the rental market. Landlords desire the best of all tenants but settle for the best they can find, and renters want the best of all apartments but settle for the best they can find. Each invests in the apartment (the landlord may paint it the tenant’s favorite color; the tenant may install permanent decorations), so each would be harmed if the other suddenly terminated the agreement. If the tenant could leave for a better flat, the landlord would have to bear the costs of an unrented unit and the search for a new tenant; he would have to charge a high rent to cover that risk, and would be loath to paint. If the landlord could evict the tenant for a better one, the tenant would have to search for a new home; she would be willing to pay only a low rent, and would not bother to keep the apartment in good shape, if she had to expose herself to that risk. [… T]hey protect themselves by signing a lease that is expensive for either to break. By agreeing to restrict his own freedom to evict, the landlord can charge a higher rent. By agreeing to restrict her own freedom to leave, the tenant can demand a lower rent. Lack of choice works to each one’s advantage.

Marriage laws work a bit like leases, but our ancestors had to find some way to commit themselves before the laws existed. How can you be sure that a prospective partner won’t leave the minute it is rational to do so […]? One answer is, don’t accept a partner who wanted you for rational reasons to begin with; look for a partner who is committed to staying with you because you are you. Committed by what? Committed by an emotion. An emotion that the person did not decide to have, and so cannot decide not to have. An emotion that was not triggered by your objective mate-value and so will not be alienated by someone with greater mate-value. […]

“People who are sensible about love are incapable of it,” wrote Douglas Yates. Even when courted by the perfect suitor, people are unable to will themselves to fall in love [… T]he […] musician is not typically singing about drugs, sex, or Satan. He is singing about love. He is courting a woman by calling attention to the irrationality, uncontrollability, and physiological costs of his desire. I want you so bad, it’s driving me mad, Can’t eat, can’t sleep, Heart beats like a big bass drum, You’re the only one, Don’t know why I love you like I do, You drive me crazy, Can’t stop lovin’ you, Ain’t nobody can do it to me the way you can, I like the way you walk, I like the way you talk, et cetera, et cetera.

Of course, one can well imagine a woman not being swept off her feet by these proclamations. (Or a man, if it is a woman doing the declaring.) They set off a warning light in the other component of courtship, smart shopping. Groucho Marx said that he would not belong to any club that would have him as a member. Usuall people do not want any suitor who wants them too badly too early, because it shows that the suitor is desperate (so they should wait for someone better), and because it shows that the suitor is too easily triggered (hence too easily triggerable by someone else). The contradiction of courtship—flaunt your desire while playing hard to get—comes from the two parts of romantic love: setting a minimal standard for candidates in the mate market, and capriciously committing body and soul to one of them.