Because
1) Hojin cant handle the fact that the man he loves would give up his dream for him. He thinks that yoosung will regret it in the future
2) grandpa said to yoosung: be in the successor seat and never see Hojin again OR can quit this job but you will never see Hojin again (like he will do something to him).
ps: yes Geom is gorgeous in those pictures! I needed to share them haha