'What is it to love another person?' This is to raise one of the deepest, and most puzzling, questions we can put to ourselves. Love is a central theme in the autobiography we each write as we try to understand our lives; but we may feel that we become only more confused the more we reflect upon it. Love is closely connected with our vision of happiness; yet there is no one we are more likely to hurt, or be hurt by, than the person we love. If love...