If you do something too good, then, after a while, if you don't watch it, you start showing off. And then you're not as good any more.

The remark warns about the moral and artistic danger that can follow genuine excellence: once a person becomes aware of being “too good,” the focus can shift from the work itself to the performance of being good. “Showing off” suggests ego, self-consciousness, and a desire for recognition—qualities that can corrode sincerity, discipline, and craft. The quote implies that maintaining quality requires vigilance and humility, because pride can subtly replace purpose. In a broader Salinger-like register, it also gestures toward authenticity versus social display: the moment an action becomes a bid for admiration, it loses the very integrity that made it admirable.