4. He Brings Out the Best in His Companions
The Tenth Doctor wouldn't be half as awesome without his companions. Not only do Rose, Martha, and Donna serve to bring out the best in the Doctor, but he also knows how to highlight the very best in them. The Doctor sees Rose's potential from the beginning, and by the end of series 2 (and most definitely with her return in series 4), Rose is a strong, intelligent companion who knows how to connect with the Time Lord unlike anyone else. Over the course of her time in the TARDIS, the Doctor teaches Martha to trust herself and her opinion. And without the Doctor, Donna would've never known that she was the most important person in the whole of creation, and thus would not have trusted herself as the woman who would save the Doctor, all his friends, and the planet.