Songfacts®:

• Here, singer-songwriter Ella Henderson teams up with Nathan Dawe to outline the reasons she can't let go of her lover, even though he sometimes drives her crazy.
• "21 Reasons" samples Danish DJ and producer Rune's 2003 dance tune "Calabria." Alex Gaudino famously sampled the track four years later for his hit "Destination Calabria."
• Dawe is a huge fan of Rune's song and played it in just about every set he's done from the age of 15. "I've always wanted to do something with that record, and I really wanted to do it justice," he told Official Charts. "I didn't want to rush it. I think we finally found the perfect concept to fit the sample around - here's 21 reasons why I love you, I think that's very relatable."
• 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.