I'm not sure happiness is that simple. It's "easy" to fix depression in someone if they're well fed, in good physical health, and aren't overly concerned about their physical safety (I'm not trying to minimize depression here, just pointing out it's relative problems). However DPRK soldiers apparently aren't fed that well, lack access to good healthcare and above all they'd be fighting on a battlefield seeing death around them and in constant fear of their life. It might be possible to amp up the soldiers, but the Germans also drugged their troops and even military leadership during WW II which didn't work out so well for them in the medium term.