Glinda begins by saying that, while she doesn't know if it is true that people come into each other's lives for a reason, "I know I'm who I am today because I knew you."
Similarly, Elphaba tells Glinda that "whatever way our stories end, I know you have rewritten mine by being my friend."
Schwartz commented after the show premiered that for the opening of the song, he asked his daughter what she would say if she would never see her best friend again, and her answer became the first verse of "For Good".
[1] Vulture ranked the song as the third-best from the Wicked score, describing it as "not particularly inventive", "comically heavy-handed", and almost overtaken by "flowery schmaltz", while ultimately concluding that the earned emotion that the show has built through the duo's relationship can "excuse all manner of lyrical sins".
[2] Playbill ranked it the seventh-best Stephen Schwartz song, deeming it less flashy than other Wicked songs, unique, powerful, meaningful and an insight into how relationships make people grow.