Is it just me?

I'm just gonna wait for this to be complete before continuing, this is too frustrating for me

It’s part of his backstory that was somewhat told in earlier chapters and more in later chapters :(

Because that's just what happens when you're put on the spot, you don't necessarily think of every good thing you could say to make things better.
Also for the whole "not being a big deal" thing, it seems to me that most of you are being completely tone deaf to what Juheon has been expressing throughout the manhwa. The main reason why he masks himself to other people is to shield himself from opening up to the wrong person. He fears and hates people who only approach him for their own benefit so he distance himself from everyone to avoid being hurt. So it's really not hard to understand why he'd refuse to hear Dohu out. Anyone would be hurt and feel like shit if they learned that the person they were convinced loved them from the start was merely interested in them for a work related project. It just means that they were never on an equal footing in their relationship, plus the stuff about Dohu only saying he loved him when he was asked to, with all of this I really don't get why so many people are being dismissive of Juheon's pain.

Nicely put, I too think their reactions were very much in sync with the personalities they've been showing up until now. Personally, I think people tent to take Dohu's side in the comments because most people go with their initial reaction, which again for most people is to put themselves in the shoes of that character and think about how they would react in that situation. but they don't really have the same perception of life as the character, since both characters are not the cookie cutter normal slice of life types, instead they come with either quirky personality and or bad pasts (to put it nicely). and as it happens, Dohu is the MC, we spend most of the story in his head, we get to understand him from his thoughts directly, but we observe Juheon from further afar. so it's harder to put yourself in his shoes without inserting your own perception of life and your own beliefs into the reaction. even I can say that, in Juheon's situation, I would have been a lot calmer and listen to dohu, but my trust in him would be shattered because I personally hate not being on equal footing with that other person and learning that I've been acting delulu would be a blow to my pride. but that's the thing, I also don't really have any trust trauma or much fear of being vulnerable, which Juheon clearly does. I for one think it's proof of quite good writing that so many people have initially the same reaction as Dohu, failing to understand Juheon completely, because we've been on his head for so long and for many chapters now we've been hoping that the argument will pass smoothly, but for Juheon is a much more raw and imidiate reaction.

Anyway I hope my little rant makes a bit of sense, i think I rambled too much, but I just meant to say I agree with your point and I think people's views will change fast one they get the time to think about it more and once we get more of Juheon's backstory