There is something about the welcome touch of one human toward another.
Whether is is the acknowledging hand on the head of a child, the hand of a friend reaching out to your forearm, elbow, or upper arm, or the embrace of a close friend, there is something about touch that transcends a simple definition.
I've read that it is about a transfer of energy.
I've read that it is about allowing another into your personal space.
I've read that it is subconscious (to both) communication (that is amazingly obvious) to an onlooker.