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Doctor Trio in YouTube pointed out that this facial expression shows that he can't be a priest. I didn't understand it first and thought it may be a thing that only guys can understand. However, rewatching this scene made me realize that this face was him being happy and relieved
I'm not that good in Hangeul but when Ikjun said he wanted Gyeoul to be happy, I think his words literally mean: "I hope it works out for her." When he added, "with my friend Jeongwon," it implies that he hopes things work out between them.
In Jeongwon's POV, as someone who was happy and celebrating Gyeoul receiving a proposal from someone, it doesn't make sense for Ikjun to suddenly say these things that he wants things to work out between them when they both know she already has someone else.
Then, it might have hit him that either the bouquet thing was a prank or the guy pursuing Gyeoul doesn't stand a chance. Either way, it implies that Gyeoul still has feelings for him. This was his facial expression as he looked up:
For me, it looked relieved and hopeful, even wistful. Relieved that she still likes him and his worries about someone stealing her away isn't going to happen. Take note that this is how he looked like as he nodded when Ikjun said he wanted Gyeoul to be happy:
He looked sad, like he resigned to his fate that she's moving on and he needs to let her go. I'm thinking that he thought Ikjun was telling him to give up his feelings for her because she already has someone else. Which may also be the reason why he was quite defensive at first.
I can see a bit of a smug smile here, which just disappeared when Ikjun sat down. He was happy with the implication that he still has her heart.
After Ikjun assured him that she still feels the same way, he went on to encourage him to follow his heart. He also didn't try to deny his feelings for Gyeoul at this point. He might have thought it's unnecessary because it seems like Ikjun can see through him.
For me, Jeongwon's dilemma is never about acknowledging his feelings. He knew about them since ep 6 or even earlier. What's the problem is that he thought he should hide it. That if he ignore it, it will fade away soon. That it may just be a passing emotion that could disappear.
I also think there's guilt in play here. Here he is, trying to become a priest and then suddenly he realized he fell in love. It's not like he suddenly fell in love. Along the way, he allowed himself to be bothered by her. He allowed himself to be curious of her, to observe her.
At some point, there was a conscious part of her that allowed his feelings to grow from mere curiosity to full-on romantic feelings. At some point, he allowed himself to pay attention to her. He didn't guard himself early enough to stop any possible development.
He might have felt a bit of guilt because no one has entered his heart as it should be for 40 years. He was able to build a wall between him and the women in his life which allowed him to work professionally with them and still not be attracted with anyone.
He slipped up. Suddenly, there's a hindrance. He allowed himself the luxury of paying attention to a woman for the first time and he didn't stop it on time. Like a coal ember, once it was placed with coals, fire would spread until it's hard to stop from spreading.
Gyeoul said he is a man of principle. He wanted to be a priest and despite the hindrance (love), he wanted to see it through. This is where his principles and desires/feelings don't agree. His heart wants to be with Gyeoul but his mind, his principles are telling him he shouldn't
