How does Natalie's death contrast with Al's according to tris

Though Tris has rightfully pondered over the fine line between bravery and selfishness, Al's death falls much further into the realm of the latter. In contrast, there was nothing selfish about Natalie Prior's sacrifice; she did it for Tris's sake, and was sure of herself the entire time. In her mother's purposeful sacrifice, Tris has at last seen a representation of real bravery.