The relationship between lovers is no exception.
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.