Yujin's father targeted Aaron again as soon as he was released from prison. Even while serving his sentence, he had managed to use his network of loyal henchmen on the outside to sabotage Aaron's car. The plan was set during his time in prison. He had known about Yujin and Aaron’s relationship from the very beginning, receiving constant updates on their every move while behind bars. After his second attempt on Aaron's life failed, he publicly revealed his identity on live TV news as Yujin's father and as the man who had previously tried to kill Aaron.
This successfully destroyed Yujin's public image, realizing Yujin's deepest fear that his presence in Aaron's life would ultimately ruin Aaron's legacy as a brilliant actor.
Yujin had been a massive fan of Aaron since he was just a child, making their connection all the more meaningful and making his father's relentless interference even more painful.
Yujin is terrified of his father, not only because of the genuine threat to Aaron's safety but also due to deep-seated childhood trauma. He reacts with visceral discomfort whenever anyone touches his neck, though the exact reason remains unknown. He is also afraid to sing in front of others and can barely finish a full song, the direct result of being forced by his father to entertain guests in adult entertainment venues as a child.
Yujin has been exposed to mature and predatory environments from a young age and Yujin's bond with his father is broken beyond repair after his Mother has died.
So in summary, IMO Yujin left for/because :
1. Aaron's safety
2. Aaron's legacy
3. His unresolved trauma
By the way, you probably need to re-read this from the get go, many things to unpack (▰˘◡˘▰)

But seriously tho, WHY HE LEAVE HIM??? When obviously he love him ? I mean yeah pretty sure his anxious and all. But mainly bcoz of his father I presume And What is wrong with his father? Is he some kind of like a president or shyt for him to be scared〒_〒 cuzZz I just remember what's with the runaway arc again ? He ran for WHAT? 〒_〒 I started to love the runaway arc at first but looking back I don't even know the reason lol or I just forgot?