Jae had this misconception that Tae brings him bad luck because when they were younger, whenever Tae runs to him or insists on seeing him bad luck seems to happen.
The one time Jae refused to see Tae (at the hospital) Jae did actually got kidnapped. Taht's when he realized that Tae dispels his bad luck, his gilsangcheon. Tae has this uncanny ability to sense whenever Jae's about to get into trouble that's why he tries to find him, and when he does, his presence negates or at least reduces Jae's misfortune. That realizarion also mwntally burdened Jae coz he thought Tae will hate him because of their dynamic. That's why he tried to symbolically cut the red string and stop relying on Taeui for luck.
Not quite understand? Taeui came see him then misfortune appear? But why Taeui consider bring luck?
Is it the misfortune was destined to happen to the brother, but only bcs Taeui make sure to see him before misfortune happen that he was lucky enough to survive all those misfortunes? (⊙…⊙ )