First of all apologies for not being able to memorize the names properly and not putting them here. And beware of some spoilers!!
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According to the raws, teahwa had some traumatic experience when he was a child. Trauma seemed to cause because of his trusted friend who seemed innocent and later betrayed him. So from my pov he developed a mask in order to protect himself being heartbroken by close people. So trust issues with people is probably because of that. And it was mentioned that he changed in high school drastically, so prolly again some trust related issues happened. But I think that teahwa somehow developed feelings to that guy(sorry forgot his name) and somehow it scared him and something happened between them which made teahwa disgusted by himself first of all, yes I mean he might started hate himself and after to cover up that truth he might show disgusted emotions towards that guy.
And so they met again, having questionable relationships (fuckbuddies) and that guy prolly thinking that he is receiving this relationship mutually as a receiver and thankful to Taehwa for not pushing him anymore. And Taehwa prolly thinking that he is doing some kindness to that guy, but in a reality he himself might be enjoying this relationship but may realise it only later when something dramatic happens between them.
Oh yes and, I've seen some guy accidentally caught them having $€% from far and he somehow looked like Taehwa's friend from childhood who betrayed him. So some drama prolly will be caused by him.
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My theory how they became loving couple, prolly happens after someone(Taehwas childhood friend or the fuckbuddy guy who strangled green hair guy earlier in previous chapters) threatens them for being fuckbuddies or even leakes out proves, and Tehwa might get freaked out and afraid that situation can repeat as in the past, but I hope that the situation between them will not be like between Yahwi and Jooin. Cause Taehwa seems more mature and knows what he wants, his relationship with his brother and friends is vivid prove that he is not a total assh@le. So yeah that's all for my creepy theory
And sorry for my bad English.