Cas isn't sure if Dean can actually go weak in the knees, but his hands come up to his arms to support him just in case. He does it before he can really think about it, pure instinct, protecting Dean, making sure he is safe. It's like riding a bicycle, only it's accompanied by the nausea of knowing he has failed in this for far, far too long. It's something he will never forgive himself for.

It occurs to him that Dean thinks he's probably dreaming this, or imagining it. Lord knows what he's had to do to keep sane these last years. So, Castiel considers the situation a moment before doing something he doubts very much he would have done in any of Dean's imaginings. He pulls him closer and wraps his arms tightly around him hugging him as tightly as Dean had hugged him when they had been reunited in purgatory.

"You're not dreaming."