I think they both sense some form of gratidute in this scene. Brian because Justin came back to him (and it was so obvious that he wanted him badly throughout early S3) and Justin for not completely loosing Brian over what he called the biggest mistake of his life. However, if Justin told Brian "I am so lucky you took me back" Brian would probably rolled his eyes. I agree that they both sort of admit their fuck ups and you can tell that they are doing things differently from their reactions to each other in late S3. As for forgiveness none of characters thought it was necessary to articulate such a thing. Justin didn't ask for forgiveness and Brian didn't offer one (most probably because from his POV he didn't think that Justin had to apologize for going after what he thought he wanted).