How much needless despair has been caused by a series of biological mismatches, a misalignment of the hormones and pheromones? Resulting in the fact that the one you love so passionately won't or can't love you. As a species we're pathetic in that way: imperfectly monogamous. If we could only pair-bond for life, like gibbons, or else opt for total guilt-free promiscuity, there'd be no more sexual torment. Better plan - make it cyclical and also inevitable, as in the other mammals. You'd never want someone you couldn't have.