Holden is afraid of the point where the joy and innocence of childhood is lost, and the only way adults seem to be able to regain the freedom and happiness they once had is to fake it - be 'phony', in Holden's words. Holden's happy childhood seems to have ended with the death of his brother. Any attempts now to be happy seem 'phony', unless he is with Phoebe with whom he can be his real self. Holden realises that as Phoebe grows he will lose this connection. Both of them will slide into 'phoniness'. Hence his desire to be 'the catcher in the rye' and preserve the last of the innocents.