Now, Stanley can no longer think of the other children as m…

Now, Stanley can no longer think of the other children as moral agents because any attempt to understand their values will be "negative thinking" that he has to repress. He can never genuinely be friends with them because he can't think about what they would like. The only thing he can do now is neg them by talking about the piano. Anyone around Stanley will understand that the piano is more important to him than they are. This can make him into something enticing, addictive even because the other children will see it as a challenge to win him over. However, ultimately they will find him boring and unsatisfying.

This is how a costly signal hurts you if you don't have any real value to back it up. Stanley will just drain himself and never get any real satisfaction back. He will be driven by a great hunger that he cannot acknowledge that can never be satisfied.