Castiel heard the distress in Sam's voice and his heart ached from it. He reached out a hand, placed it on Sam's face lightly. He know humans tended to find that a comforting touch. He hoped that Sam did.
"Calm yourself," he said softly, sitting beside Sam to show that he was staying. He couldn't offer a great deal of real comfort. He couldn't promise Sam it wouldn't happen over and over. He kept his distress at this from Sam.
"I can. Until we find a solution you may put this on me. It is not to heavy a weight." He left his hand on Sam's face, kept his gaze on Sam's. "Tell me what you need and I will do it." Sam was suffering all of this for the world. He had given up himself to set things right. Castiel, knowing this, couldn't leave him to suffer alone. He would help him find that illusive peace as best he could.