Making decisions on your own is hard enough, but when other people get involved and try to pressure you one way or another it can be even harder. Peers influence your life, even if you don't realize it, just by spending time with you. You learn from them, and they learn from you. It's only human nature to listen to and learn from other people in your age group.
Peers can have a positive influence on each other. Maybe another student in your science class taught you an easy way to remember the planets in the solar system or someone on the soccer team taught you a cool trick with the ball. You might admire a friend who is always a good sport and try to be more like him or her. Maybe you got others excited about your new favorite book, and now everyone's reading it. These are examples of how peers positively influence each other every day.
Sometimes peers influence each other in negative ways. For example, a few kids in school might try to get you to cut class with them, your soccer friend might try to convince you to be mean to another player and never pass her the ball, or a kid in the neighborhood might want you to try out shisha or bhang with him.