I like "It Would Be So" because it is so poignant.

It is a song that expresses the regret and longing left behind after a breakup in a calm yet poignant way.
It flows calmly at first, but the more you listen, the deeper the emotions become, leaving you with a heavy heart.
The members' delicate vocals bring out the wistful atmosphere of the lyrics even better, making you fall in love with the song.
The atmosphere is particularly strong in evoking memories of past love, so if you listen to it late at night, the lingering feeling lasts for a really long time.
If you like emotional breakup songs, this is a great song by DAY6 that you should definitely listen to at least once.