Yeongwoong has liked Maru for a long time, which both Maru and Doeul noticed. Maru started to like Yeongwoong but was afraid that their friendship would be ruined or that he'd hurt Yeongwoong if one day he stopped liking him (and so kept their relationship as just friends). Doeul also liked Yeongwoong and unlike Maru was finally willing to act on his feelings, but their relationship was purely physical since Yeongwoong still liked Maru (though this new development has been confusing for him). Maru realizes what's happening with Yeongwoong and his brother and isn't sure what to do (I think that's why they seem to drift apart at this point). Maru doesn't want to lose his friend, so he tells Yeongwoong he knew he's liked him all this time and says he likes him too. I think Doeul realizes that while Maru cares about Yeongwoong, his confession was just about saving their friendship as opposed to Maru truly having romantic feelings (that, or he isn't sure if his feelings are still there and is using Yeongwoong right now to test those feelings). This is why the Brothers fight - Doeul is warning Maru that by exploring his feelings he may end up hurting Yeongwoong and they will end up drifting further apart than they were before. Also Doeul does still like Yeongwoong and is trying to protect him, but also seems to be trying to respect Yeongwoong's feelings for Maru....at least that's what I think is happening >w<
Aight y'all time to grab yo knives and pitchforks Σ(っ°Д °;)っ