In her Spring Day reaction, Yunyi felt BTS' tactic of having a sad song in a major key (major keys usually give off happy vibes) was similar to some of Schubert (a classical composer)'s pieces.

"Schubert was arguably the most famous and most influential for his lieder (German art song)! It's a common misconception that all classical music is either instrumental or operatic. His song cycles Die Schöne Mullerin (The pretty miller-maid), Winterreise (Winter journey), and Schwanengesang (Swan song) all have incredibly tragic narratives but comprise of a number of incredibly unsettling major selections. These major pieces appearing in a gloomy subtext provide a haunting feeling of irony and delusion, which I think Schubert particularly excelled at and is relatively infamous for today. For example in the narrative climax of Die Schöne Mullerin, there's this one song titled "Mein!" that is this triumphant trumpeting tune in major about how the singer has finally earned the miller-maid's love, but it turns out to have only been his imagination.... The rest of the song cycle after "Mein!" is centered around the decay of the singer's sanity. I thought that same idea of "foreboding major" appeared in a lot of BTS's stuff, especially Butterfly and Spring Day. It's really cool and a really hard and complex effect to achieve, so good for them!"