He can criticise him for taking so long to get it right, but it's very satisfying to see him tell her what's up finally.
I like that the conclusion is that when it comes down to it avoiding small discomforts isn't worth endangering something as big as one's relationship and Ri-na is really that important to him. It feels like a pretty realistic struggle, because it's easy for a teenager (or an adult) to get caught up in all the small things and miss the big picture, so it's nice to see it depicted here so clearly. It is simultaneously a very silly mistake for Park Dong to make and at the same time a really deep lesson for him to learn.