Because dahm was in his first year of college so it was hard to meet up and he noticed that because sunwoo had been visiting him his grades were bad so he broke up
from what i remember, yoo dahm(the white haired guy) broke up with sunwoo (the black haired guy) because yoo dahm noticed that sunwoo didnt have any motivation to do well in school anymore (after yoo dahm moved out for college) and the only thing he looked forward to was to meet him after school, so yoo dahm thought that the best option was to break up so sunwoo can focus on school. (im not sure tho, sorry if im wrong)
Oooh, i get it now, but...8 years that's a little too much
Why did they break up?