Because Shiyoon's solace was someone else at that time, but then he got betrayed by this person (most likely his ex? Iirc he's shown in some school flashbacks and we definitely saw them have sex). From how manipulative they're relationship seemed to be, I'm certain that person was the one who isolated him mentally and pimped him out.
During and after that issue he continued to fall further down, unable to come to trust or open up to anyone else. Doggo and maknae most definitely tried to get him to open up, but since he pushed them away super violently, they seem to have grown to not be pushy and only show care subtly, like in the beginning of the series.
Still don't get how the yellow hair get strength from og MC yet he can't do the same for him.