In romance, Tolstoy’s aphorism about the family is reversed. All unhappy couples are alike, and all happy couples are happy in their own way. Happiness in a couple is a private and fathomless world, a far cry from the mere shared sensibility of the happy family; we can only make fun of the impish, impenetrable languages of other couples, which exclude us. Yet we all know what it is to be unhappy i...