Here is some background information on "Next Girl" from Songfacts.com.
Patrick Carney was in a really bad place, in the tail end of a crumbling relationship. The story goes that Dan Auerbach wrote this tune for Carney to acknowledge the fact that he knew what his partner was going through with the divorce. "Oh I wrote that song before," Auerbach told Mojo magazine: "It wasn't about him. I wrote that song a year and a half before we recorded Brothers, around the time I was working on the solo record stuff. .