In Chapter 14, Bruno realizes that he knowingly has talked about Shmuel to Gretel. He understands that the implications of this action. In feeling bad because the rain keeps him from hanging out with Shmuel, Bruno has tipped off Gretel of his friendship. Bruno considers telling Gretel the truth about Bruno and "coming clean." Yet, Bruno recognizes that this might be more trouble than it is worth, and realizing that Gretel might react unpredictably to it, Bruno ends up telling her that his friend is imaginary.
Bruno considers telling the truth to Gretel because he sees nothing wrong in his friendship with Shmuel. Yet, Bruno is developing the understanding that the world might not see it the same way and Gretel, recently becoming infatuated with the world of Nazi power might be a part of this world. He decides against telling her the truth and comes up with the cover that Shmuel is his imaginary friend. He knows that he has been successful because Gretel ends up questioning his maturity and makes fun of him.