"Castiel meets the Doctor for the first time three weeks before he raises Dean from hell. Cas appears in front of him, expressionless- something twists in the Doctor's gut at that. "Who are you? How do you know about us?" Castiel takes a step towards him. The Doctor smiles wryly. "Angel of Thursday, yes?" Castiel doesn't react and the Doctor is struck by the fact that yes, he is an Angel of the Lord, he isn't Cas yet. "Human?" Castiel asks. The Doctor sees a flash of his wings. "No. Are you?" -FA"

Sometimes Castiel traveled with the Doctor just so that he can travel the universe even as he can feel his power draining. Aside from Dean, the Doctor is the only person who really understands Castiel.

When he started working with Crowley he avoided the Doctor. He knew his friend wouldn’t approve. Then one day a woman, another Time traveler, found him. Her name was Melody Pond.

“He needs you.” She said, “His greatest battle is being fought at Demon’s Run and he needs you.”

“I can’t.”

“Yes you can, because he can help you.”

“I don’t need his help.”

“Oh, yes you do. You need each other. You’re both old and infinite, seeing so much of the universe and traveling with human companions to be your conscious at your side. Come with me.”