Why does the Doctor favor and protect Earth, as he is a Time Lord and not from Earth? Is there any canon explanation?
There are a couple of other explanations: - in the TV movie the Doctor states that he is half human on his mother's side. So thus if that is true and his mother is from Earth's future, then by protecting Earth he is simply ensuring his own survival.
IIRC, in the Doctor Who RPG, it is also stated that the Earth is a "temporal nexus point", meaning that changes on Earth have bigger effects on the wider universe, although it is not really explained in any more detail.
And an additional one - we know from episodes like "The Doctor Dances", that the human empire eventually spreads out throughout the entire universe. So destroying the Earth before that happened would leave a void which could be filled by another race.
There are two major reasons that The Doctor favours Earth.
First, the Doctor has, at various points, praised humanity on it's survivability, indomitable spirit, and explorer's spirit (reference). Even for the episodes that don't take place on Earth, there's usually humans around. Additionally, most of the Doctor's companions have been human. So, the Doctor has likely developed some level of emotion for humanity.
Second, and perhaps more important, and relevant to the new series, the third Doctor spent a period of exile on Earth, being unable to leave for other planets or times, and working with UNIT to defeat various threats to Earth. This, combined with the destruction of Gallifrey, means that Earth is like a second home to the Doctor. That's why he feels it's worth protecting.
Bonus out of universe answer: there's definitely some amount of bias here, as Izkata said. If the Doctor went around praising and protecting some other species, the show probably wouldn't be as interesting or accessible to most/all people.
Here are a few quotes that may explain it.
TIME LORD: We have accepted your plea that there is evil in the universe that must be fought, and that you still have a part to play in that battle.
DOCTOR: What? You mean that you're going to let me go free?
TIME LORD: Not entirely. We have noted your particular interest in the planet Earth. The frequency of your visits must have given you special knowledge of that world and it's problems.
DOCTOR: Yes, I suppose that's true. Earth seems more vulnerable than others, yes.
TIME LORD: For that reason you will be sent back to that planet.
DOCTOR: Oh, good.
TIME LORD: In exile.
DOCTOR: In exile?
TIME LORD: You will be sent to Earth in the twentieth century, and will remain there for as long as we deem proper, and for that period the secret of the Tardis will be taken from you.
-- The Doctor admitting that Earth is more susceptible to evil in "The War Games"
"What's the matter, old girl? Why this compulsion for planet Earth?"
--The Doctor talking to the Tardis when it randomly choose Earth, yet again, as it's destination. It would seem that the TARDIS is following the orders of the Doctor's release and Exile. [Episode, Black Orchid]
"I am just a tourist. I like it here."
--This may be just be a snarky comment, but it's also possible that he is just exploring all of Earth history (perhaps partially against his will, thanks to the Tardis), and fighting evil when he comes across it (as ordered).
The Doctor is something like a thousand years old. What makes you think he spends most of his time helping Earth?
We probably only see the stuff that's relevant to us. Self-centered humans, indeed...
I have no evidence. But, is it possible, that the Time Lords exiled The Doctor to Earth to basically imprint him into saving humanity because humanity is the future of Gallifrayan/Time Lord kind?
Time Lords survive till the end of time supposedly. The Doctor always says that humans survive, they always survive. Maybe, just maybe, humans are the Time Lords. Perhaps this one small thing was kept from The Doctor. Humanity must survive, because people matter. Humans matter - as do the Time Lords.
I believe there was an episode - I have actually been trying to find it - where the Doctor is asked why he favours Earth. He answers that he was in a bad place in terms of his emotional state after he pretty much destroyed his and the Dalek races when he happened upon Earth. He goes on to marvel at how it felt like home to him.
Firstly because Susan, the Doctor's granddaughter in the beginning wanted to go to Earth for an unknown reason, also from reddit, r/gallifrey Do we know why the Doctor left Gallifrey?:
Just wondering if i missed the real reason he left Gallifrey.
derf82 • 3y
I thought it was implied that when the Doctor was a kid, he got lost in the cloisters and heard the prophesy about the Hybrid before the Wraiths showed him the way out. This freaked him out for some time (i.e. the scared boy in "Listen"). Eventually, scared that it would apply to him (perhaps because he is half human, so Me implied), he stole a TARDIS and ran.
At least that is now my headcanon. Of course, all we have is his statement he ran because he was scared.
CountScarlioni • 3y
This freaked him out for some time (i.e. the scared boy in "Listen").
I think Listen pre-dates him breaking into the Cloisters. He hadn't yet been accepted into the Academy by the time that we saw him in Listen, and in Hell Bent, he says that it was a student at the Academy who broke in.
oodja • 3y
Here's my hybrid theory: the Matrix showed him how bored he'd be if he stayed on Gallifrey, which scared him into leaving.
113-171-154-145-104 • 3y
That makes sense i suppose, but if he did see the prophesy about the hybrid why did he wait so long before running away.
derf82 • 3y
Perhaps he wanted to or even tried, but couldn't yet operate a TARDIS. Or perhaps if he is half human, and didn't find out his true parentage until he was much older.
LegoK9 • 3y
We just found out he left not because he was bored, but that he was scared, of what we don't know yet...
07jonesj • 3y It had to be something to do with Susan, I feel. People seem to forget that he didn't leave Gallifrey alone.
I loved what they did with the character in Big Finish, but I would like the character to have a heavy involvement in the show at some point in the future. It's one of the only parts of the Doctor's early history I think could be covered well without destroying his mystique.
CountScarlioni • 3y
In Heaven Sent he confesses that he didn't leave because he was bored, but because he was scared. Scared of what? Good question. We learn in Hell Bent that the Cloister Wraiths spoke to him when he was a student at the Academy, and the idea put forth is that they told him about the Hybrid. Is that why he ran? Maybe. Maybe not. By the time that he meets Ashildr at the end of the universe, it seems as though his concept of the Hybrid could have been just as mistaken as any other.
There have been a number of reasons suggested throughout the show's history. In Resurrection of the Daleks, he says outright that he left Gallifrey for reasons similar to those that caused Tegan to leave him (she could no longer bear the lifestyle). Remembrance of the Daleks suggests that he wanted to hide the Hand of Omega, for some reason. Robot of Sherwood implies that he left because he wanted to help those in need, and couldn't stand the Time Lord policy of non-intervention (which is broadly consistent with the Second Doctor's "there are some corners of the universe" quote).
If you watch Peter Capaldi, Patrick Troughton, series, of the show you might understand more.