What is so sad about the interaction between Ender, Alai, and Shen? Do you think Ender is still isolated?

Ender's promotion proved his apptitude for leadership but changed the dynamics of his friendship Alai and Shem. Things have "gone sour" in that Ender has respect but no camaraderie with his old friends. They treat him now as a commander and teacher but not as one of them. Thus, he has been effictively isolated.