The Doctor had doubts. That alone made him an outcast. His people never doubted. The Time Lords were utterly convinced of their place in the universe, secure in the knowledge of the task appointed them -- that they had appointed themselves. The strict control and protection of time was their sacred duty. They were the authorities of the universe, answerable to no one.
Like the rest of his kind, the Doctor answered to no one too, but unlike them he had doubts. He didn't let his human friends see them; they had enough to worry about. But when he was alone...