I won’t let any of you die.
Ah this moment is so important. Misha totally killed it with his expressions here. Sure we can think that Cas is relieved that the Winchesters are reunited with their mother. But his sorrow is evident. He longs to be a part of that but still feels he is an outsider to it.
Cas’s entire current emotional arc is revolving around his sense of belonging and finding a place in the world to call home. We know thanks to 12×03 that he doesn’t believe he fits in with the Winchesters even though he “cares about [them] too damn much” but this moment and his speech afterwards proves that even though he feels he doesn’t fit with the Winchesters, that he wants to.
This is so so important because before this season we have really seen Cas going back to heaven and trying to convince his siblings (and himself) that he is still a part of heaven. He has finally turned his back on that and this look proves it. He sees the Winchesters having their group hug and he wants in.
This is Castiel expressing a want, a need, a desire for the family he has found. We now know exactly in canon that Castiel has chosen the Winchester’s officially. He’s done kidding himself. He wants to be with them, but he doesn’t yet feel like he is worthy for it. He doesn’t believe that they would want to include him yet.
This is where his emotional arc needs to go in the coming episodes. We need to see some serious work done on the Winchesters part to include Cas and fight for him (and having two Cas episodes coming up its looking promising).
Basically, come season end, I want another scene like this. I want a moment where the Winchester’s recoup after a battle and go for a hug, I want Castiel to stand aside like this and watch them with a look of longing on his face, but the difference is that this time, Dean turns his head, looks at Castiel, reaches out a hand and pulls him in. Then Sam comes and wraps a giant moose arm around him too and that would be that. He finally accepts that he is indeed family and a Winchester in his own right.