
I profoundly agree with you. To me, Jisuh and Heejae have some inner demons that they have to deal with. They both carry baggage. Sunyool, has some, but it’s not at the depth as the other two, or the type is too different. Jisuh and Heejae, their demons play well together and their baggage kind of forms a matched set. They fit together very well, because of that. Moreover, they both approach things very intentionally, whereas Sunyool is far more whimsical. Sunyool could be a good balance to Jisuh, if he had experience dealing with his demons and baggage and still managed to be so carefree. But he’s just too “young,” for lack of a better word. He just doesn’t have the experience of dealing with those problems, yet, and so he wouldn’t be able to properly support Jisuh. Heejae can. He won’t get it right all the time, but it will be better than Sunyool trying and failing all the time.
(⌒▽⌒)
That’s just my take on it, though.

It’s not a matter of knowing, it’s a matter of “reading between the lines,” for lack of a better phrase. If you pay attention to how he approaches work (he only got a job to be near Jisuh, rather than for his own ambitions in life). His getting the job is entirely on a whim. Leaving school is on a whim, which he admits to. Even approaching Jisuh is on a whim after he realizes Jisuh and Heejae have feelings for each other. I’m not saying that he doesn’t have struggles or that he doesn’t feel deeply or even that his feelings are fake. It’s more like he has had support and people to rely on in facing his struggles, which affords him the luxury of whimsy. He has openness. Jisuh has no one, and Heejae has his sister who told him to hide in front of everyone, except for her. They don’t have the safety net that Sunyool has. That is partially the source of their reservations and a sense of coldness. They fight their demons on their own, whereas Sunyool doesn’t have that practice yet. Just look at how he handled getting kicked out of his place. He went to someone else to solve his problems, and then he propositioned Jisuh, because he doesn’t take Jisuh’s feelings for Heejae seriously. We, as readers, know what a big deal it is for Jisuh to admit his feelings, but Sunyool doesn’t get it, because he’s never been in that position before. My overarching point is not that Sunyool doesn’t have issues or the potential to face them, but because of his inexperience, he lacks the wherewithal to face his issues on his own, and he lacks the wherewithal to support Jisuh in doing the same. Speaking from experience, that makes communicating incredibly difficult, and it breaks down relationships pretty fast. When someone doesn’t understand your inner demons, or that you even have them and why, someone can’t accept your darkness, and thus can’t accept all of you.

OH! And silliness isn’t necessarily what I was talking about. People who are the silliest tend to be the saddest. It can be an excellent coping mechanism. Sunyool isn’t just silly, he’s quite innocent in how the world works for those less fortunate or for those who are alone and without a support system. He has the capacity to understand, but just hasn’t learned yet. If he had truly understood, he wouldn’t have propositioned Jisuh after knowing his past. I think that’s part of the reason Jisuh got pissed, and also why Sunyool seems confused that he’s pissed.
Am I the only one who ships Jisuh and Heejae?
Consider this, both Jisuh and Heejae are most likely the type of person who deep think into a relationship. They are both adults who have lots to loose if shit goes wrong, and they most likely have equal emotionally turmoil experience as a closeted gay. That's why their cold and aloof personality really matched each other.
They are very different to Sunyool' flighty characteristics and tendency of being a cute-pie pushover. Notice how Jisuh only became 'slightly attracted' to Sunyool when the kid made an aloof like expression on the beach. Or maybe is Jisuh words, looks like a snake.