Taylor Swift told Jimmy Kimmel:

It’s a song I wrote that is loosely based on… I wrote it with my friend Jack Antonoff (of Fun. and Bleachers) who’s dating my friend Lena [Dunham]. Jack sent me this song, it was just an instrumental track he was working on and immediately I knew the song it needed to be. And I wrote it as a kind of commentary on what their relationship has been like. So it’s actually me looking and going and ‘this happened and that happened then that happened’ and that’s how you knew, “You are in love”.

This song along with “Wonderland” and “New Romantics” are only available in the deluxe version of 1989, which was exclusively sold at Target.