I'm going to tell you this... you're misreading the situation. He's NOT doing this to hurt Bum or torture him in any way, in fact, to him this is going to make them closer. Remember how he said it's their "anniversary"? I've read ahead and yes, it does bring them closer, albeit in a very morally twisted kinda way...
Because yoonbum accidentally sits on another guy's lap and it made sangwoo got jealous .. i think..
Okay, I know he doesn't want to hurt (well, maybe loose him is what he's afraid of) Bum, but anyway I think he's kinda testing Bum. He also did in he store and when he hanges Bum and said "let's se if you keep kalling my name" (or something like that). I feel like if he's testing Bum's love.
hanged* calling*
Sangwoo is really in love with Yoonbum. But i really don't understand why did he do that on ch. 17. (╯°Д °)╯╧╧