Why shouldn't you ever tell anybody anything chapter 26 of Catcher in the Rye?

Holden Caulfield makes this statement because he is disillusioned with phoniness and dishonesty in society. He believes that people will twist or manipulate information to suit their own needs, so he chooses to keep things to himself to avoid being misunderstood or betrayed. This reflects his overall cynicism and alienation from the world around him.