Theories abound as to why cold or temperate climes produce happier people than warm, tropical ones. My favorite theory is one I call the Get-Along-or-Die Theory. In warm places, this theory states, life is too easy; your next meal simply falls from a coconut tree. Cooperation with others is optional. In colder places, though, cooperation is mandatory. Everyone must work together to ensure a good harvest or a hearty haul of cod. Or everyone dies. Together.

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 future payback. There is a word for this: love.

Excerpted from The Geography of Bliss by Eric Weiner. Reprinted without permission from Hachette Book Group, USA.