I think both felt happiness and relief to be back together. Justin was lucky that Brian took him back, but but I'm not sure "grateful" is the right word for how I interpreted their feelings, as emoted by the actors. Relief, yearning, etc. Justin looked triumphant and happy, and Brian looked sheepish (and happy), really, in that picture, like he was embarrassed that he was about to take back Justin, that he'd been beaten at his own game (again). :)