Sam is Castiel’s friend, insofar as Castiel understands the concept, which he suspects is imprecisely. They have shared meals, or at least Castiel has been present during the consumption of food and alcohol. They have discussed strategy and tactics. Sam has confided some of his conversations with Lucifer, though Castiel knows that it is likely that Sam has held something back. Sam wanted to know if Lucifer could bring him back from suicide; Castiel was forced to assure him that it was within Lucifer’s power. He thinks he has dissuaded Sam from the attempt, and he is glad. Castiel doesn’t wish Sam to suffer yet another death and resurrection, the terrible disappointment of an escape averted. Such an experience could only bring Sam closer to saying yes to Lucifer as a way to bring an end to his pain.
Now Anna has her own plan, digging in the dirt of the past, pulling the infection in the Winchester blood out by the roots.
Anna is as clever and puissant an angel as Castiel has seen this side of the archangels. Still, she is not powerful enough to defeat Lucifer’s plan, Lucifer who sat by the Lord’s side and knew the tricks of seeing even a sparrow fall. Anna will not be able to change the course of time, will not prevent the conception of Sam Winchester any more than she will be able to prevent Azazel’s baptism of blood.
In the past, Lucifer has not yet risen. He may not, as Anna and Castiel may, move freely in the lives of Dean and Sam’s parents.
If Sam dies in the past, Lucifer will not be able to bring him back. Not immediately, in any event. Though it is not entirely certain, Castiel deems it at least likely that Lucifer’s powers of resurrection would not work on a corpse thirty years in the grave. And there are further measures Castiel might take, if he can recover from the journey in time: burning the bones, scattering the ashes.
Dean would react badly.
He would blame Castiel, which is a trivial concern in the larger apocalypse, but Castiel is after all a fallen angel, and to fall is to become interested in one’s own happiness, if no more able to achieve it.
They are desperate. Dean in particular despairs, unaided by Castiel’s faith that his Father may still intervene, unstrengthened by the rage that, however unevenly, sustains Sam. Dean grows brittle, and Castiel would rather have him full of hate than empty enough to surrender himself to Michael.
So when they urge him to bring them all back to confront Anna, Castiel does not point out the danger. Dean’s mind is alcohol-blurred, kept from concern by his default assumption (almost always correct) that Sam is safer with him than without. Sam—Castiel suspects that Sam may know the risk he is taking, if he even regards it as a risk instead of a boon. For all that they are friends, they are not close enough for Castiel to ask that question.
Castiel will be unable to protect Dean. This is a risk he will have to take, because being unable to protect Sam—
Is an opportunity he can’t ignore.