David Brooks starts The Social Animal with a marriage and a baby. This lets him describe marriage as a "map meld," where two people gradually meld their maps of the world together.
The map meld, in turn, lets Brooks describe the minds of mammals in general as growing through "mindsight." We intertwine our lives with the lives of others. This applies to husband and wife, and to parent and child. We learn to feel what others are feeling by mirroring their actions in our minds – even if we do not mirror them in our bodies (this is the theory of "mirror neurons"). We mirror what others are feeling by interpreting the meaning of their actions.
I especially enjoyed Brooks' contention that humor is a tool we use for bonding with other people, and is itself the reward for getting our minds in sync with theirs.