Have you ever listened to the odd sound that a fax machine makes when it is trying to connect with another fax machine? There is a weird series of whirs and whistles. This series of sounds is called a handshake. Machine A is trying to figure out what frequency machine B is on so it can send the fax correctly. When they are able to properly connect it is the same as two people shaking hands.
When people shake hands they are communicating in a basic way. At a primal level they are saying they aren’t holding any weapons. Simply touching another person is a big deal. We have a lot of rules about personal space. It is seen as rude to get too close to someone. People stand about a foot and a half away from each other in line. But when we shake hands we have to get closer than that.
Just offering to shake someone’s hand is a big deal. They can refuse. Or they could want a hug instead. There is a bit of jostling about to figure out what the other person wants out of that interaction.
What about a wave, or a smile? Have you ever noticed that if you wave “hello” to a stranger they will invariably wave back? The same is true for a smile. Remember the saying “Laugh, and the world laughs with you, cry, and you cry alone”? Substitute “smile” for “laugh” and you are on to something. Try smiling at a stranger today and see what happens.