In my opinion, art(be it movies, music, etc.) is intended to make the viewer feel emotions. Think of your favorite movies/songs, and ask yourself why is this my favorite? The reason is mostly because of the way the movie made you feel. Your favorite movie(s) would have evoked emotions in you that you strongly relate with. So whenever you want to feel those emotions you will want to watch the movie again :)

This movie made me feel super-emotional: sad when I saw the background stories of the main characters, and happy when they were able to find happiness and love in each other.

At the surface level, the movie seemed "light". It's less than 2 hours long, and the narration doesn't spend a lot of time to set up their background. However, it looks like they didn't need to, as the acting and screenplay is brilliant that they are able to emote really well and make us go through what the characters are going.

Especially in the end when Jeong-hwa is running to find Cheol-min after she finds out he's visited her after 2 years. Can't find him and eventually breaks down in tears: It's hard not to feel emotional there.

Han Hyo-joo(the lead actress) has such a beautiful smile, it's so hard to not get sucked into the character and fall for her when she emotes with that smile and innocence.

So Ji-seob(leading actor) doesn't show a lot of emotions on his face, because his character doesn't reveal them to the outside world, instead internalizing his pain and his past. But you can still feel what he is going through.