I've been thinking:
Jiri was in a really bad place (and was very unfair) for most of the story and Yuri pushed his own issues away because he wanted to support and take care of the teacher. Now that he's doing much better, Yuri is being childish and showing his ugly side. But I think Sunwoo is very grateful to him and knows that now it's his time to be there. Sometimes he humours Yuri but other times he's helping him navigate their relationship.
I just didn't like it because I was expecting just happiness in the extras. I feel like this could've been another season where we explore Yuri's feelings and insecurities.
The End