i doubt it's about him having feelings for the other dude but more of Seunghyun realizing it was a major part of his life, That his feeling did waver to somewhere/someone else.
it never was resentment toward the manager (Because he was the one dating the other guy), But also realizing/facing the person, the ''reason'' seunghyun had almost a decade of this unrequited love for someone.
It's hard to accept to turn a chapter of your life like that, when it was such a big deal for him. It was a type of tolls/shackles for something that he never could let it out. (he hold himself to tell a confession and in the process, did not get a clear cut rejection and turn a page in his sentimental life)
It's hard to accept something so strong (in that case feeling of Love/Longing) is now over, cause you don't know if it will ever comeback.
IMO: Seunghyun was just startled by the fact that he was indeed maybe not in love anymore with the other manager for a long time. And being with the manager just spooked him about that realization. ¯_(ツ)_/¯ that's my take
Love this!!
"First Love", especially an unrequited one, can be very powerful and affect a person in some way for the rest of their life (try asking an older person about their first love). Seunghyun being involved in the purchase of that pen, was itself a major event that affected his life. And on top of that, finding out there's a connection between Kim Jongchan and his first love was a shock. It understandably threw him off.
The problem was that Kim Jongchan didn't understand all the turmoil that was going on in Seunghyun's head (he's never experienced anything like that himself so he has no reference), and he walked away because he thought if he couldn't understand it, then he couldn't fix it, so why bother?
Now he realizes, he doesn't need to understand it. He just needs to be there for Seunghyun and be reassuring until Seunghyun works it out that what happened in the past doesn't matter. And then they can stop eating heavy pasta. And have sexy times.
ah sorry for not making it clear, i understood that part but if i was the section chief, my train of thought would definitely go to where i said in my comment "why is he still hung up?" cause Seunghyun's explanation was very limited. thanks for still explaining though!
thank you so much! this makes me more confident on how i understood the situation
thank you for the responses! i was not so sure about how i processed his emotions and up to now i still don't fully understand how he feels, this is also majorly because i havent experienced falling in love just yet (might be shocking since im already in my 20s). i did think he was stuck on the past, still feeling the pain of being in an unrequited love (even though he doesn't feel anything for yoosung now). now that he's trying to move on from that, he gets reminded of his past again. but my major question was why he was stuck on that feeling in the first place? this goes back to me not experiencing that yet, and hearing your detailed explanation helped me realize how deep a first love must really feel like, which again i can't really grasp since i don't have experience. thank you so much.
err.. i still don't get seunghyun's feelings about the matter tbh... i hope we get enlightened on that the next chapter. i dont think the section chief was the only one at fault. if i were to be the recipient of what seunghyun said about his unrequited love, i'd really feel upset. "why is he still thinking about his unrequited love? why is he still caught in it? it's been years... does he still have some feelings somehow??" if it were me, those thoughts would flood into my head... I don't understand, and it'd hurt to hear those especially that the only explanation seunghyun's given that time was "it was a problem that the section chief was just tangled in."