Written by Henderson, John Nicholas Ealand Morgan, Maegan Cottone, Dawe, Rune Reilly Koelsch and William Martyn Lansley, with production from Dawe and Punctual, "21 Reasons" features a prominent sample of Rune's 2003 dance song "Calabria".

Ella Henderson previously collaborated with Nathan Dawe on the song "Lighter" with rapper KSI.

Dawe said that despite the single not being out, during an early set people were able to sing the lyrics back to him when he played the song.

[7] A week later, "21 Reasons" was charting alongside the song "Je M'appelle" by London drill music artist Benzz.

At the time, both of Henderson's collaborations were still charting, "Crazy What Love Can Do" reached a new peak of 12, while "21 Reasons" sat at number 14.

[12] At the time of the song's release, Dawe confirmed that a music video had not been shot but said he was open to working on one so long as he was not dancing in it.