My good opinion once lost is lost forever.
Darcy states a proud rule for living. The absoluteness is revealing—it protects him from disappointment but also from growth. Austen later tests this rule until it softens, showing that good opinion can be earned back when truth emerges. The line explains some of his stiffness and social missteps. It also sets up the pleasure of watching certainty yield to empathy. The novel suggests that lasting regard thrives on humility.