Unpopular theory tho he probably has feelings for Mokyeon and perhaps tried to make him jealous with Nanjo, not expecting Mok would fall for him. After all Mok's friends said Sunwha was still holed up in his room when they visited him and that was after Mok showed Sunwha's messages to Nanjo so I doubt Nanjo was still blowing up his phone after learning Sunwha was just using him.
So basically Sunwha is depressed and doesn't go out anymore after his falling out with Mok. It could be that he's just upset about their friendship but he seemed to be upset to find out Mok really liked Nanjo.
Off topic but I like Sunhwa's character lol he's a bitch but he KNOWS he's a bitch. Guy has standards and immediately left the moment he saw Nanjo and Mokyeon. I'm just glad he wasn't the type to cling to Nanjo
It'd be funny if he helped the two get together though lol