Adolescents Think They Know Everything
Adolescents would like to believe they know everything do. They think they know everything. This might make you chuckle, but you only laugh because you know it is true. Adolescents, in reality, are insecure, unaware, self conscience. They know very little about themselves and even less about the world around them. It's easier for them to deal with their ignorance (lack of knowledge and understanding) by pretending that they know everything. This is totally normal.
Adolescent think they have all the answers to avoid being vulnerable, and because of insecurities its hard for them to admit that they don't much. They are attempting to become independent and to do so they feel that they must prove that they can handle it. When they admit that they are ignorant, they are not instilling confidence in the adults around them. Therefore, the answer is to "fake it".
"Know It All's" Look Foolish
The truth is that none of us know everything, and those pretend to know everything look foolish. For most of us we don't like to feel vulnerable, and its intimidating to be caught looking stupid. This situation is intensified with adolescents. Adults who do no a great deal need to exercise patients and understanding with kids who act as though they "know it all". To get into an argument with a "know it all" teen is not profitable for anyone.