All Castiel was trying to do was save the world. And to save the world he had to hurt his friends. Did anybody actually think that he wanted to break Sam's wall? Dean had forced him into that. Dean wouldn't see it that way, though. No, Dean would blame him for breaking Sam. Dean always blamed other people when it was usually the Winchesters who were at fault. They started the Apocalypse in the first place. If Dean hadn't broken the first seal things would never have gotten to this point. But Dean never saw it that way. He blamed the angels and Lucifer.