Songfacts®:

• Lewis Capaldi wrote "One" with his friends Phil Cook and Tom Mann after one of them showed him a poem. The verses were written to a girlfriend's ex-boyfriend who had treated her badly. They thank the dude for the detrimental effects his bad behavior had on her as it led to the writer of the poem and the girl getting together.
  
  I guess somebody else's loss is another's gain
  I'm sayin' thank you to the one who let her get away
  
  Inspired by the poem Capaldi wrote this ballad thanking a guy for bungling the relationship, which resulted in her running into his arms.
• The majority of Capaldi's songs on Divinely Uninspired to a Hellish Extent are weepy breakup ballads, but "One" is an exception. The Scot told OfficialCharts.com he was excited about the track as its "actually a love song as opposed to all these depressing, heartbreak songs."