You can tell she loves this song so much. It's in every single concert playlist. Even if it makes her cry, she always sings it because it means something to her. The sings it so goddamn PASSIONATELY that it drives her to tears. One day I'd love to hear her talk about this song and its connection to her emotions.
A great singer is not about nice make up and fancy clothes, but the way how she conveys the emotion through a song. This song is sad, but it is even sorrow because of Taeyeon.