ok spoilers ahead!!
I believe the sad part starts when jiwoo and dongchan spend the night in the cabin/storage in the woods, they wake up because the cabin caught the fire, they made it out eventually but jiwoo goes back to retrieve the painting dongchan painted for him, but he can't escape and dongchan can't get to him so he runs for the help, jiwoo is saved later on but also is in bad condition due to a heart disease - that's the angst in this story. Don't wanna ruin the experience for you by adding too much details, but eventually jiwoo leaves and they lose contact untill graduating. Like I said I don't want to spoil everything, but the story ends on a happy note so go for it:)
everyone’s talking about the last 10 ish chaps being rlly sad/they start crying because of it can someone tell me what happened before i start reading so i can mentally prepare myself…spoil all you want !!