because he was losing it. It wasnt because Seungho wanted out of fun, he was indeed concerned about his well being and the only way, during the time line of this story, is to slap him to snap out of it. Seungho wouldnt shout at him, as that wont help. Slapping nakyum was the only way to realize he went out of line.
On the other hand, i would prefer that he held it together than saying Hold me to Seungho. I have a feeling Seungho likes Nakyum because Nakyum doest want him. Im not sure about Nakyun's move, it seems he is defeated and just giving in.
Yyyy no?
1. He didn't need to be slapped.
2. Seungho slapped him because he raised his voice to him.
chapter 56 showed both of them vulnerability with each other, letting go of pride for love. Too much to take in, very early in the morning