All the great ideals that gave life meaning in earlier societies – God, the nation, revolution, freedom, democracy – are in disarray today, questioned by many and rejected by those who have lost faith in them. But there is another value, rooted in the birth of the modern family and in the passage from traditional to modern marriage, that has transformed our lives in profound and often unrecognized ways: love. It affects not only our personal lives but many aspects of our social and collective...