I suppose people who know themselves well are thought to be mature (in comparison to those who don't know themselves too well, anyway). But I'm only beginning to realize how much we not only define ourselves by knowing who we are, but by the choices we make. Of course we base our choices on who we know ourselves to be, but can't we also base them on who we'd like to be?
I guess it's one thing to want to be somebody else, and another to want to be yourself, but better.
In Harry Potter and the Chamber of Secrets, Harry asks Dumbledore, why the sorting hat did not place him in the Slytherin House even though many of his skills are so like Voldemort's. Dumbledore's answer? "It is our choices, Harry, that show us who we truly are, far more than our abilities".