I really like this smile, because it’s Dean smiling in spite of himself.
Because he’s standoffish with Cas from the moment they run into him. Because he KNOWS ‘Zeke’ will disapprove of Cas hanging with them. So he doesn’t want to get too attached to having Cas around, doesn’t want to start to enjoy and be happy about the fact he’s getting to spend time with Cas again, since he knows it can’t last.
And similarly, he doesn’t want to give Cas the impression that they are going to be a team, doesn’t want Cas to get comfy and invested in them all hanging out together, because that will only cause Cas to be hurt more when he is inevitably asked to leave.
But then Cas is SO ADORABLE. And being such a GREAT GUY fighting the good fight. And the three of them feel SO RIGHT together.
And so Dean CAN’T HELP BUT SMILE, just for a moment. He just CAN’T HELP but let his true feelings about having Cas close again out.
You can even see in the way his brow furrows and the smile is that little bit strained that he’s FIGHTING the feeling and the smile, I think. Because he believes it’s pointless to be happy about Cas, to enjoy being with Cas, when he’s only going to have to give that happiness up (a lesson he learned when he had to give up Robin - don’t invest in someone/something else, because you’ll only have to give it/them up in the end, because his responsibility to Sammy always comes first).
But despite all that, despite telling himself not to care and not to show he cares, his love for Cas shines on through anyway!