Dawe previously worked with Henderson on his 2020 single "Lighter
." In that instance, her vocals were heavily filtered, but this time it's her real, unembellished voice.
Speaking about how Henderson became involved on "21 Reasons," Dawe said they had kept in touch after "Lighter," and upon finishing the demo, Henderson came to mind. He wanted "proper Ella" not "Chipmunk Ella" this time.
Within a minute of Dawe sending the demo over, they'd set up a Zoom and the pair wrote the lyrics in two hours.