Namwoo said that he already made peace with his feelings like even before he and ML met. After his confession (after highschool) he ended that feeling and it just turned to a lingering regret. After years of not seeing each other when he heard he is getting married it just opened closed wounds and painful memories. He already made peace and happy for them but it just hurts. quite relatable.
It doesnt look like it bec it is just 6 chapters but Namwoo said ML has been clinging to him for months (not days or weeks). So the feelings were not rushed. Of course 6 chapters cant show every interaction in the past months. I wouldnt see any reason for him not to fall for ML during those times coz he is no longer dwelling. It will also easier for him to love the next person more bec this time he feels it's not unrequited unlike before (well before he learned about Yeorum's identity). So when Yeorum 'fessed up if Im in that position hearing how this person was in my position before and his feelings is towards me why wouldnt I like them more? That's how I see it hehe
Which is why I said it's rushed. Six chapters can't possibly really encapsulate all the bits and pieces of information and feelings of the characters.
I am going to go against the grain here and say this was super rushed and really not as good as this author's other works. When did MC have the time to fall for ML? He was in love with someone for 20 years and suddenly he likes the ML out of knowhere. I genuinely can't imagine him saying "I like you" to Yeoreum like he did to the brother.