Don't take this the mean way, but like others said he is aromantic, so yeah he doesn't feel the way you described it and that's totally understandable.
The real problem is that he never communicated this to yoomin in their entire relationship, like he never really cared for her.
(And no I will not take something like "What if he feared that yoomin would leave and such." Because i think yoomin would have accepted it and tried to work around it, because she loved him.)
I'm aromantic. I get it. He is not an aromantic his a platonic romantic. And duh I agree.
Aromanticism is a spectrum (I'm saying this as someone who's aromantic)! You can feel romantic attraction, and be aromantic, it's not that simple. In my opinion, I'd say he's either aromantic, or alexithymiatic.
I don't need to be learned sweetness I know. My comment is an umbrella for a much deeper convo that I will not talk about under my own comment. Which ever the case is he falls within an area of a spectrum that does fit the spectrum of 'societal normal' love. In simple terms.
YOU DONT KNOW WHAT LOVE IS?? ILL TELL YOU!!! ITS AN UNENDING CRUSH!!!! THAT'S WHAT IT IS!!! IF YOU DID NOT KNOW NOW YOU FUCKING KNOW!!! LOVE IS A PASSIONATE CRUSH THAT USUALLY CHERRISHES SOMEONE TO THE PUREST AND HIGHEST DEGREE OF RESPECT AND DEVOTION.
Go to Hyundo. The only man. That loves truly and genuinely. Go save him.