How does peer pressure occur?

Peer pressure is a state of mind. Although for whatever reason, there are mental illness's that can be classified under the cause of peer pressure. I think peer pressure is all about the similarities all of us have in common. We each see ourselves as part of the herd. Social living has influence on us all. This is exemplified in the statement by John Donne ' No man is an Island'. Compare Paul Simon's song 'I am a rock.....'