What are the common causes of peer pressure?

In the case of peer pressure, a person is convinced to do something (such as going to an Opera and doing illegal drugs) they might not like but which they perceive as "necessary" to upkeep the positive relationship with the other party, such as the family of their partner. The person could agree to the offering even if he hated it for many reasons; maybe he or she is the possible inheritor of the family/person that asks him or her to come to the opera, the family could want to evaluate the person before letting their son/daughter get married with him/her, etc. from wikipedia