The answer: mirror neurons. They put us inside the body of others, making us literally feel what they are feeling. Mirror neurons explain why we shrink back in our seats with fear when the hero is abruptly attacked by Freddy Krueger in A Nightmare on Elm Street and why we soar with joy when young bicyclists life into the blue sky in E.T. The Extra Terrestial.
These neurons are kicking in when we wince after a kid tumbles off a swing onto the ground and when we break into a smile watching a friend's eyes light up as we carrin in a huge birthday cake.
This ability to enter into another;s experience is especially pertinent in love relationships, in which responding in a sensitive way to a partner's needs is so central. When we see our sweetie's mouth droop down or eyes well with tears, our brain mimics the experience for us.
In a sense, we physiologically try on the feeling. The line between us and our partner blurs, and we automatically without conscious reflection or deliberation, feel and know he or she is sad.
This is invaluable in helping us tune in to a mate and in building intimacy, safety, and trust--the very bond of love.