We follow arbitrary norms that offer no rational reason for us to conform to them. The norms can snowball when we are influenced by people's earlier decisions.

People tend to conform to behaviors that are common among other people, even when they know that those people did not make their choices freely, and when the decision does not mirror their own desires. 

Increasingly, social norms are being used to encourage pro-social behavior. They have been successfully used to encourage:

The idea of the self-categorization theory is that people conform to the norms of certain social groups whenever they have a personal desire to feel like they belong.