The Romans kissed to greet one another, and one Roman emperor ranked a person's importance by the body part he was allowed to kiss. Those of importance were allowed to kiss his face, lesser nobles were allowed to kiss his hands, and others were to kiss his feet. In Europe, the British are probably those who have held back most when it comes to kissing. Southern Europeans were kissing friends publicly long before the British considered the idea. However, today women also find it natural to kiss a good friend on the cheek, as a way of showing that we feel a close relationship to him or her.
Men are still reluctant to kiss a member of the same sex.
Kissing makes us feel good, and because kissing leaves us with a positive attitude to life, it's actually healthy for us.