Sometimes we think the love of another is all we need to make ourselves whole, and we invest everything in that relationship. In this song, Goulding has a catharsis as she realizes that she was whole all along; the guy who left her "was never the missing key."
Goulding wrote the song with Oliver Goldstein, Bonnie McKee and Cory Nitta; Goldstein and Nitta produced it. In a Songfacts interview with McKee
, she explained: "It's about co-dependency and having an epiphany about that. Realizing that all you really need is yourself and belief."