Why is peer pressure a Strong force in a teenagers life?

I believe it has always been this way and isn't a new thing. It stems from a simple need to 'belong' and be accepted.

To give an example, if you study the social habits of prisoners you can see a lot of 'tribal' behavior, a new inmate will immediately seek out his own kind in order to belong. When I say 'own kind' I mean the people who display the closest affiliations with his/her own social background/ethnic group.

It's the same with teengagers and because they are still maturing and developing their own personalities they are more easily influenced by other people.