Jun said that he would break down when he could no longer accept what Zhang Hei did to him, but it was Zhang Hei who broke down first after he heard the young man's words.
It was as if he had received a hard slap on his cheek which made him realize his mistake. He couldn't even be angry with Jun because the young man was only stating the truth today. He was the first person to draw Jun closer to him until they committed to a special relationship, but then he suddenly wanted to let him go.
Unfortunately, it turned out to be a cycle as he ended up wanting to pull the young man closer to him once again. If it really was a cycle, then there was definitely a day when he wanted to let go once again, and maybe that was the reason why Jun wanted an explanation before his heart was hurt once again.