The main reason is that he promised Tiancheng Cai Yi a promise before, but he didn't keep it on the stipulated date, so he kept it from Tiancheng Cai Yi. This story comes from 19 episode.
Tiancheng Linyin and Tiancheng Cai Yi are the characters in the comic book Idol Dream Sacrifice, which tells the story of "Private Dream Academy" with 37 students and two teachers in three grades. Because Tiancheng Phosphorus Sound is Tiancheng Cai Yi's brother, Tiancheng Cai Yi always bothers his brother to do many things for himself, and Tiancheng Phosphorus Sound chooses to avoid him in order not to disturb his brother.