This song was written by Yoshihiko Ando (安藤芳彦), composed by Mioko Yamaguchi (山口美央子) and arranged by Takao Sugiyama (杉山卓夫). From what I understand of the words, it concerns the female subject pondering over her uncertain feelings towards someone. Musically speaking, it's an intimate and elegant composition tinted by melancholy. I found the whole piece quite powerful in a nuanced way. Can't pinpoint the exact reason why. Her delivery reminds me of a French singer Elsa Lunghini, who also performs a similar kind of gentle and feminine music. And speaking of French, there are a couple of French lines in the lyrics and the way Harada pronounces the title sounds like "C'est la vie". I wonder if that was intentional.
Now, hopefully this doesn't come off as a spoiler, but I was quite taken aback by the soft jazz outro when I first heard it. You don't see it coming judging by how the rest of the song is but somehow it feels quite satisfying, almost like a kick into the material world after several minutes of internalized sorrow. Either way, it works.