"I want to be able to protect you and Sam," Cas mumbles, pressing his mouth to Dean's knee. "I want to be able to heal you." If he was thinking less selfishly, he'd add that he wants to make right everything he did wrong and save the world again, but he isn't. Right now his focus is narrowed down to two points, Dean and Sam, with an emphasis on Dean. Save Dean. Protect Dean. Care for Dean.