Because communicating is hard—it's easier said than done. Ongoing fears can hinder communication. Like, for example, Joon doesn't want Doha to worry because he sees it as "unnecessary," and that's preventing him from saying he's having a hard time.
This! But also Joon has faced repeated rejection from Doha specifically, so now that he finally is in the position he's always wanted to be in where Doha accepts and reciprocates his love to some degree, he's incredibly anxious not to screw it up. He'll do just about anything to try to make sure Doha doesn't feel pressured or stressed in their relationship, but he thinks the best way to do that is to put himself down and aside, because as we've seen, that's what's worked in the past. That's how he got to where he is now.
YES, EXACTLY!!!
why cant joon just open his mouth and communicate...