Dean's going to need time to adjust to this, he's going to need to see the situation for himself. Castiel knows this, and yet he wishes Dean could take him at his word that his brother is safe and well.

"I can no more stop Sam from doing something than I can you, Dean. You don't understand, Sam is no more a human soul now than you are. No more a human than I am an angel. We'll go see him and you can judge the situation and speak to him yourself. Please believe me when I say though that this is not a punishment for him.

If you want my honest opinion? I think it was intended as a reward."