I believe he started having feelings for him and got scared! That's how it usually goes!
The younger brother would do everything the woo yeon would do and be better at it than him. I think he thought the younger brother was being arrogant. Also the writer should've written about it much better, but it's hinted that the younger brother is a large part of the reason Wooyeon had no friends in highschool. He turned them all against him, which led to people bullying Wooyeon. Even the secondary male lead says "Wooyeon told me you could never tolerate other people being near him."
to be honest i dont care if they get tgt, but i need to know what did the seme do during they childhood that made the uke personality do 180... bcs if i rmbr correctly in one of the chap it say that they used to be close until smt happened. I NEED TO KNOW