'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 is something we all want, why is it so hard to find and harder to keep? Love is one of humanity's most persistent and most esteemed ideals, but it...