Is your friend a true friend?
Some kid's "friends" or peers put pressure on them to make them do something that they know isn't right. For example, some teenagers start smoking because of peer pressure. Little kids also have this sort of peer pressure. Imagine this class clown on top of the monkey bars with other kids surrounding him and pressuring him to fall of the bars head first. Do you think that this is safe for a kid of any age?
Scared or Confident?
Peer pressure makes bad things look "cool'" or "popular" so then the kids loose their confidence and start making wrong decisions.