(I copied this from another comment that explained it extremely well @tintin00)
The reason Yujin ran away is because of the guilt and fear that his father would harm Aaron and would make Yujin as his pawn. After the Christmas tree incident, Yujin's mom lost her cognitive function and mistook Aaron as her son and she ran out into traffic and got run over which affected Yujin's dad so such (he is devoted to her) that he decided to abandon Yujin and kill Aaron and he did it almost and so it starts to weigh down on Yujin's conscience. After that car accident, Yujin was planning to leave to ensure Aaron doesnt get hurt, but for a while things went smoothly and Yujin reconsiders staying with Aaron but after that news report, Yujin went into mental breakdown thinking that it would harm Aaron and is blaming himself for Aaron getting hurt by his dad. Aaron knows this but he does not blame Yujin for it and begs him to stay by his side. But the guilt and pain ate away at Yujin and so he runs away.
The news comes out about yujin father attempted murder Aaron when he's a child, yujin feel guilty because he's wasn't being able to tell the truth sooner, even tho Aaron said he's fine with it, and yujin father seems not give up to kill Aaron now even after release from prison, and rejected the deal to stay from yujin that proposed by Aaron, yujin left for Aaron safety sake.
Thanks so muchhh
Guys i cant remember why yujin ran away. Can someone give me a refresher pls