4. She Gave The Doctor A Reason To Live
After the Time War, the Doctor was very alone. Alone in the entire universe. He could feel it in his soul, that there were no Time Lords left. So not only did we have a lot vested in his new companion, but he did too. There was no half-assing this one: his companion had to be fantastic. Rose was fantastic. The Doctor adored her, and was willing to live to keep traveling with her. Not only that, he was willing to die for her. Dying is no small thing to the Doctor, and 10 was really upset about it. But 9 accepted it and went smiling, because he knew he had saved Rose, and that made it okay. He didn't get wrapped up in the pain and fear like 10 did, because he had someone there for him, waiting on the other side. And when he came back, he smiled at her.