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That played a role in why he kept everything secret and tried to push him away, but he ran away because he didn't know what to do....simple as that. His BF is "sexing" some guy he saw him with before. In the flashbacks he even says Dong betrayed him before, so he is going with what seems logically to him: he got betrayed again.
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He didn't even know Dong was getting bullied, and when he did find out he rant to help him. Dong then said he wanted everything Yuri loved to die along with him. Yuri THEN ran away. He tried to call him days later, so technically he came back for Dong.
I KNOW WHY YOU ARE HATING YURI BECAUSE OF WHAT HE DID, BUT I THINK IT'S NORMAL FOR HIM TO REACT THAT WAY. HE WAS AFRAID AT WHAT MIGHT HAPPEN NOT ONLY TO HIS SELF, BUT TO JEARIM AS WELL( SAW THE FLASHBACK?) . IF HE EVER ADMITS HE IS DATING JEARIM, JUST IMAGINE HOW MUCH WORSE THINGS WILL GET IN JEARIMS END,LET ME REMIND YOU GUYS THIS IS NOT YOUR TYPICAL YAOI WHERE THE SEME CLAIMS HIS UKE INFRONT OF EVERYONE ELSE AND EVERYTHING WILL GO SMOOTH AND HAPPY ENDING. IN REAL LIFE TOO MUCH FVCKS WERE GIVIEN IN SITUATIONS LIKE THAT. THIS MANHWA DEPICTS HOW JUDGEMENTAL THE FVCKIN WORLD CAN BE AND HOW MANY ASSHOLES ARE THERE TO RUIN EVERYTHING, JUST TO SATISFY THEIR OWN GREED (╬ ̄皿 ̄)凸