They are waltzing around each other. Kind of a cat and mouse game, where Siwoon (Woo Jin) is obsessed with Siwon (Tae Min), because he perceives/feels/sees Tae Min in him, so, he's purposely aggressively pushing Siwon's buttons so to speak, to provoke him. The more Siwon reacts, the more Woo Jin recognizes Tae Min in him. Siwon on the other hand, is purposely being aggressive trying to keep Siwoon at a distance, first, because he's scared that he will discover his secret and second, because he still harbors resentment over what he considers to be Woo Jin's abandonment. So there's a lot of anger between the two which is really based on the love they had for each other as children.
What with all this hostility between the two main characters, holy crap.