That's peer pressure. Think of all those times your friends pressured you into doing something you didn't want to do or into not doing what you do wanted to do. Especially since I know I don't need the extra calories. I ended up getting a subway sandwich with tomatoes, which I hate. Now, if someone were to let this type of peer pressure carry on, they could become someone that they're not, just so they could look cool in front of their friends.
Insecurity is another reason that peer pressure plays such a large role in our lives.
Not being sure that what you think is important enough or cool enough or whatever causes us to be insecure about ourselves. Everyone is a little insecure sometimes. It's one of the things we must conquer in life. But in order to conquer something you have to be aware of it, and most of us aren't. So the next time your friends say something that you do or don't like, speak up.