Castiel struggled to keep his faith after he watched Uriel die, knowing they can never be reunited in their father's kingdom. He could never understand why an angel as fearless and loyal as Uriel, an archangel, would betray them for the Enemy. But now as he watches over the Winchesters, having faced down Gabriel, Raphael, and Uriel, he understands some of that need to rebel- for though he is an angel, he too is tired and ready to lay down his burden.