we think like that because we are the readers and we know how much dohu likes him and that it's genuine but to juheon it could've all been faked, juheon already thought lowly of himself and he doesn't even have family to lean into. now he knows the person he loves also approached him out of interest like everyone else, I'd be crushed too
The very person he'd opened his heart to inflicted the same pain as all the others. It's not much, given his personality. He just distanced himself from what caused him pain. Isolation is both his coping mechanism and his defence mechanism, it's only natural for him to leave.
It doesn't matter how unimportant Dohu's actions were, they touched him where it hurts the most.
SPOILERS
Yeah it has to do with his family. Juheon went to his grandfather and told him he is not going to be studying abroad (if i remember correctly him studying abroad was a condition set by his grandfather to Juheon's "mother" for getting the inheritance) and tried to cut ties with his family. His aunt (ie "mother") was also there. As Juheon was leaving his aunt ran after him, slapped and yelled at him for what he did but Juheon wasn't budging. Then she went well you are not leaving me with any other options and pulled out a picture of Dohu. So basically Juheon got blackmailed to go abroad because he didn't want Dohu to be put in danger because of his messed up family.
I feel like after knowing Dohu after all this time Juheon should be a bit more understanding about why he was approached initially with "intentions". In a regular situation I get why Juheon would be so upset at this and I still believe it's reasonable for him to be upset, but with all he knows about Dohu's character does it not make sense that he would not approach anyone randomly? Like I don't want to project my head cannon that Dohu has autism or something and my hindsight bias, but was it not clearly obvious that Dohu had intentions when he was initially following Juheon around?
SPOILERS
I don't know. I just feel like completely abandoning Dohu for 4 years and quitting his job and everything over this was just a bit much for Juheon to do.