Another good paragraph from Geography of Bliss: Iceland

Necessity may be the mother of invention, but interdependence is the mother of affection. We humans need one another, so we cooperate - for purely selfish reasons at first. At some point, though, the needing fades and all that remains is the cooperation. We help other people because we can, or because it makes us feel good, not because we're counting on some payback. There a word for this: love. p. 178