Love implies sacrifices. Making a sacrifice for someone you love is playing the cooperation card. A failure to make a sacrifice in return, a dishonesty or infidelity, is a defection. Sucker's payoffs in relationships include being left alone to raise the children while the ex-spouse goes off to marry someone younger. The social trappings of love, such as the engagement ring, the wedding ring, the ceremony itself, are tangible manifestations of cooperation in the game.
Love is a game that only succeeds if both parties play every round in the cooperation zone.
Marriage thrives in an atmosphere of perfect honesty and openness. Marriage is delightful when your spouse is someone you can tell anything to, discuss any problem or issue, without fear of betrayal. Jealousy implies a strong fear of defection. As in every other type of prisoner's dilemma, the shadow of the future is important. A marriage, which is supposed to be for life, is an endlessly iterated series in which any defection may provoke retaliation or end the relationship entirely.