Because it was who he was. He had a major god complex, but also what else did he have left to protect? Gallifrey was safe and yet he might not live to see it again. All he had left was Earth, the universe, and companions. And the most important of the three always end up hurt.

"I wonder if you did know just how dangerous." He turned and looked at her. "I'm not much of a doctor anymore, Donna. I forgot my own oath. I forgot how to be... me."