I don’t think we know for sure. My best guess is that Yujin was just unfamiliar with his feelings and thought it was best for him to run away and that it’d be better for both himself and Aaron. However Aaron obviously did not take it well. That’s my best guess but I don’t think we ever got a solid answer as to why he left
Yujin is afraid of his father and he's seriously traumatised because of his father. When his father made appearance in tv and came out of jail on bail he could no longer feel safe and was always anxious about what his father might do.. yujin was also afraid that he might come after aaron maybe. i forgot a bit but thats the overall thing ig.. if its wrong then please correct me

Like I don't understand why Yujin is actively running from Aaron. I don't exactly remember how the prev season ended, and I get that his dad was the one who tried to shoot Aaron as a kid, but I thought Yujin ran out of shame. Does he think Aaron wants to kill him?