I think Chae-heon's obsession came from Jeong-yoon losing contact with him like two times ...

I think Chae-heon's obsession came from Jeong-yoon losing contact with him like two times all those years. When he didnt obsess over him he lost him so he probably feels like he needs to leave a permament mark in Jeong's life so that he cant forget about his existence. Probably those years of them being vague with each other caused it ╮(￣▽￣)╭ Because it was probably so easy in Cheon's eyes for Jeong to just live his live despite him not being in there anymore, so I guess there might me some insecurity