His muscles and hands weren't the only parts of his body that had toughened over the past several weeks. His heart had hardened as well.

This statement describes Stanley's development at Camp Green Lake. Although Stanley is used to being picked on, he exhibits no sympathy for Zero when Zero reveals that he cannot read. Instead of trying to help Zero, Stanley instead decides to reserve his energy for himself. This is not purely Stanley's fault, the tough people that he is surrounded by have influenced him and now he is becoming like them.