Well this explains a lot.
Whereas self-esteem tends to be at its lowest in adolescence, and slowly increases throughout life, narcissism peaks in adolescence and gradually declines throughout the lifespan. Therefore, the development of narcissism and high self-esteem show the mirror image of each other throughout the course of human development.
I’ve always wondered why you shouldn’t trust someone who peaked in high school, and perhaps this is part of the reason.
Maybe the best people have high self-esteem, but low narcissism, and so anyone who was confident and happy in high school probably had the latter.
It would also explain why so many of them tend to be miserable later in life.