Sihwan def acts this way because sohan was the first person he chose for himself and the only one that treated sihwan with love and care as a real person for a long time he was just nothing but an experiment. he has never really interacted with anyone and if sihwan has they were only talking to him as an object plus he was created as something with no family the only purpose for him was to look like someone that is dead so he doesn't understand why sohan would ever want to stay with his shitface dad that doesn't even care about him. when sohan could just stay with him who deeply cares and loves for sohan, sihwan sees sohan's dad as nothing but a bother and a pain in the ass that is stopping them from living happily ever after together he doesnt understand how sohan feels or why he feels what he feels so he's quite obviously extremly happy about his dad leaving
Sihwan def acts this way because sohan was the first person he chose for himself and the only one that treated sihwan with love and care as a real person for a long time he was just nothing but an experiment. he has never really interacted with anyone and if sihwan has they were only talking to him as an object plus he was created as something with no family the only purpose for him was to look like someone that is dead so he doesn't understand why sohan would ever want to stay with his shitface dad that doesn't even care about him. when sohan could just stay with him who deeply cares and loves for sohan, sihwan sees sohan's dad as nothing but a bother and a pain in the ass that is stopping them from living happily ever after together he doesnt understand how sohan feels or why he feels what he feels so he's quite obviously extremly happy about his dad leaving