Because they don't have much experience in being concerned about others. Most of their experiences have been with people being focused on their well-being, parents, teachers, babysitters, coaches. It isn't until they start interacting with other people outside of home or school that concern for others can really develop.
They are worried about how other's view them. They feel the need to alter who they are to accommodate the way they think others would like to see them. If they are unhappy with themselves, they'll change [not all young people are like this].. They see themselves as wanting to be cool, part of a crowd and fitting in. They don't want to stand out.
Young people deal with a lot of emotions growing up. They're dealing with big things like making plans for the future, creating their identity (whether they realize it or not), and getting themselves out in the world. It's a "coming out of the cave" movement and it can be overwhelming quite commonly.