It started when Jihoon accidentally saved the boy from falling down the stairs three weeks ago.
Had he known this would happen, he would have let him fall and die.
When Changbin starts dating "Wooyoungie" Jisung realises two things:
1 ) Changbin isn't as straight as he thought
2) it's too late to act on his feelings.
(Or maybe not.)
[The story is pretty much completed, but needs some fix]