Okay I'm lowkey stupid so can someone explain why Yujin even left?

PLS MEE TOO BRO IM SO CONFUSED

Trauma caused by the father. Secondly, Yujin's father is also emotionally and mentally abusing Yujin, blackmailing him saying that he'll do smth to Aaron from chapters before. It's like his father programned his mind to be scared of him and come to him as dog, slave under any circumstances.