Of all the Doctors in Doctor Who, the Ninth Doctor is my absolute favorite. Christopher Eccleston served as the doctor for the first series of the rebooted version and, for the most part, made the most of what he had to work with. Eccleston was tasked with reintroducing the world to the Doctor. Not an easy task, but he pulled it off, pun intended, FANTASTICALLY.
Why did the reboot of Doctor Who work as well as it did? Well, for starters, the Doctor was now a pained war hero. Our good Doctor is (supposedly) the last of the Time Lords, who (supposedly) wiped both the Time Lords and mortal enemy the Daleks in order to save the universe. His home planet, Gallifrey, was destroyed in the process, leaving the Doctor with scars that couldn’t be healed by a regeneration.
Normally, time-aware species can sometimes interfere with events of history, but the magnitude of such an event caused time to be locked. Any attempts to “fix” history would be met with possible insanity. More importantly, the Doctor, after regenerating, felt that protecting his second home of Earth was the most important thing now.
Personally, I think he was using Earth as a safety blanket. the Ninth Doctor was extremely bubbly, almost too happy for someone suffering from the long-term effects of complete destruction of his home. Really, it takes a while for the Doctor to reveal the pain. While he briefly reveals to the Autons in Rose that “he couldn’t save anybody,” it takes seeing a surviving Dalek to really begin the unraveling of the Doctor.
Really, the only thing that keeps the Doctor from not giving up completely is Rose Tyler. Disclaimer: I love Rose Tyler. She is the companion that I started with, and like the Ninth Doctor, is my first love when it comes to the series. Rose keeps the Ninth Doctor engaged in traveling time again, so much that, when it is revealed that a Dalek fleet did survive, he did not lose control enough to simply kill everyone. He kept his cool, finding a way to save Earth without breaking his “no killing” rule again.
As the Ninth Doctor opened the door for the Tenth Doctor, I felt the usage of a regeneration to tell a post-war story was a good usage of a regeneration. I liken it to changing your clothes after a long, cold, dark, rainy day. As the Doctor came to grips with his new role as the last Time Lord, he was (storyline-wise) ready to change again.
The Ninth Doctor is an interesting character study into how war can affect the returning soldier. While his run was short, Eccleston is remembered for bringing Doctor Who back to live, and that is something that geeks everywhere should be thankful for.