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wait why did he start crying?
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This is just my opinion, but I think it makes sense. I believe Do-jin cried cause of immense guilt due to the fact that Ji Ho found out that Hyun-ui knows about the whole "tutoring" thing being a lie. Hyun tricked Do-jin, into telling him everything (base from the translation). Also if you still remember the panel where back inside the house, Ji Ho told Hyun "Why didn't I like you instead?" in front of Do-jin, where it most likely hurt his feelings, I am sure he also cried because of that, so, to sum things up shortly. Do-jin cried because he felt as if it was all his fault and was also afraid that Ji Ho might end up liking someone else because of what he had done unintentionally.
Just to share, my heart fluttered high when I saw him crying again, seeing how responsible he is to instantly admit his mistake and afraid to loose his lover so much and for the fact that he was rubbing his head to Ji Ho's head as a subtle way to ask for more kiss, gave me such a warm feeling inside. Need a man like that. T-T
pls i forgot that collar dude even existed