i guess he's not mature enough when it comes to relationships - not knowing where to cross the line and also being a little bit greedy to have everything. he feels like it's okay for him to have everything that he wants i guess.
if he's not being so obnoxious about controlling dohee, i would have been fine with him a little, because i do understand the part where he's worried about sunbae because who wouldn't be worried when your best friends got close to some unknown guy, but eh, why are you so annoyinggg