To me they both look grateful, relieved, and mostly just happy to be back together, but definitely not in the "I'm so thankful you took me back after everything I did to you" sense. As notreallyme10 said, Justin might consider himself lucky that Brian took the risk again, but I didn't get the impression that he thought he had any more to make up for than Brian did. They acknowledged their mistakes in their own slightly twisted way and moved on from there on the same page.
As for anyone telling Brian that Justin is lucky... yeah, definitely six arms and belly button on the forehead.