
Same thoughts here. I was so excited to the "chasing arc" but when I read that he KNOWS that he doesn't feel THAT kind of like towards the secretary/best friend I was kind of disappointed. 'Cuz what do you mean your heart does not flutter even once lol that just mean that he doesn't truly love him. But at the end he says he feel something he couldn't understand. So I thought maybe he's just about to discover what it is like to be genuinely in love. (I hope so)
But hey this is just a fiction, why am I over analyzing things lol. Still love this story, hope for them to have a good ending too!

I think that the fact he was so obsessed (not in love cause man you don't even know him no more) with Hajin and the whole idea of reuniting with him might have actually blinded him to any possible feeling he ever had towards someone else (not necessarily Seojoon) he got so used to only considering Hajin as a lover that he probably convinced himself he couldn't love anyone else. I do not believe that he didn't consider Seojoon as more than a friend at least once even if it might have been subconsciously and that is probably why Geonwoo told him all of that (since he saw how he always was attached to Seojoon) cause yes you can feel all that for your best friend but the moment when you realize that the idea of seeing him with another bothers you (and that seems to be the case in one of the last panels) that ain't only a friendship no more
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Also spoiler" incoming
I saw some parts of the latest chapters on IG and it seems like he might have actually had a romantic interest in him but never acknowledge it (not sure cause sadly it was only pictures without dialogue) he might have actually compared him to Hajin (even just look cause they look alike) but still decided it was only a friend cause he was oh so busy chasing his (as they say in c-dramas) moonlight and let's be fair it almost never works with childhood crushes
Dunno but being a bitter bitch I think to be actively pursuing your childhood first love and being obliviously inlove with your secretary/highschool bestfriend doesn't feel like possible. I think his just regretting not knowing it and feels guilty how his secretary could have been hurt for so long by seeing him inlove with someone else. But oh well I assume that he'll pursue the assistant and tell him he has been blind on what's in front of him blah blah blah then the assistant we'll accept his "romantic" feelings and happy ending.