guiding your teen to independence with love and humor
The theory of Psychosocial Development created by Erik Erikson is perhaps one of the best known personality theories. The theory differs from many others in that it addresses development across the entire lifespan, from birth through death.

According to Erikson's stages of Psychosocial Development, your teen is stuggling to master stage 5: 

Identity Versus Confusion

They must answer the question, "Who am I?"

Their job is to master the concept of Fidelity; the idea of being faithful and loyal.

Social relationships become their main focus.

This stage occurs during adolescence between the ages of approximately 12 to 18. Teens need to develop a sense of self and personal identity. During these teen years, children are exploring their independence and developing a sense of self. As they make the transition from childhood to adulthood, teens may begin to feel confused or insecure about themselves and how they fit in to society. As they seek to establish a sense of self, teens may experiment with different roles, activities and behaviors. According to Erikson, this experimenting is important  to the process of forming a strong identity and developing a sense of direction in life. Those who receive proper encouragement and reinforcement through this stage will emerge from this stage with a strong sense of self and a feeling of independence and control. Those who remain unsure of their beliefs and desires will feel insecure and confused about themselves and the future.