On getting Gopu Mama's chappals, the music teacher tried not to look too happy. Why?

Though the music teacher was really very happy when he got Gopu mama's new chappals but he pretended not to look happy as he wanted to show that he feels bad for his missing chappals. Also, he carried the look on his face which helped him show that his old chappals were better than Gopu mama's new chappals and he was taking those chappals with him only because he had no other option.