Swift penned the song with the Nashville songwriter Tommy Lee James
. "We wrote it about the idea of seeing someone that you used to have a thing for, and seeing them out in public for the first time after you've heard that they've move on," she explained to Spotify. "That, you know, your life is kind of in shambles and they have moved on and they're really happy. And it's almost like, even them being nice to you hurts you, because you're like, in such a state of pain and you haven't moved on yet."