I have often wondered why humans crave physical contact with each other. I think women and gay men seem to be more affectionate, but even straight men slap each other on the back, give a high five, and other such motions.

I think all of this translates equally into affection. This, I believe, is derived from parental instincts in humans. We want to take care of each other, it's how our species survives. This might also be the reason we become lonely.