It's safe here, and Castiel knows that. Knows that the tightness of Sam's arms wards him against a world that's run off its axis.

"I didn't tell you. I couldn't bear the thought of you knowing I failed again. Something so simple as being human. How do you do it, Sam? I don't... I don't understand."

He shifts a little, presses a little closer. Seeking shelter. How has he never known how nice hugs are? He got a glimpse back in Purgatory, when Dean wrapped him in a hug, but he hadn't been able to understand and appreciate the gesture back then.