On another note, the "why does the Doctor spend so much time on Earth" question was answered a very long time ago.
In the first storyline, the Doctor and his granddaughter Susan are hiding out in London in 1963, she is attending the local school and two of her teachers, Ian and Barbara, follow her home one day to try and work out why she is so strange.
The Doctor panics and kidnaps the pair, but quickly regrets it and tries to take them home. He has little to no control over the TARDIS at this point, but as he keeps trying and trying to get them back, they naturally tend to head back towards Earth, England and the mid-20th Century. The TARDIS basically gets into the habit of going to Earth every other trip. Over time, the Doctor ends up taking on other companions, usually from the same period, and their natural curiosity about the future and past of their own world, plus his great affection for the place, make it the obvious destination.
When he is finally caught by the Time Lords, they exile him to Earth, recognising that he has a deep affinity for the planet, and has friends and contacts there (The Brigadier, chiefly) who will help him settle. It's also clear that they send him there to intervene in what is suddenly a huge number of invasion attempts, and the presence of the Time Lord criminal The Master on Earth at the time may be another reason.
The Doctor spends years working at UNIT and when he leaves, it is with another Earth-based companion, Sarah Jane Smith.
His next companion, Leela, is descended from human space explorers, so he takes her back to show her what her homeworld was like.
By the time Romana comes along, he is pretty much obsessed with the place. It has become a second home for him. No wonder, when you consider he's spent a good part of the last 200-300 years of his life there by that point.
Not long after that, he picks up Tegan and spends a couple of years trying to get her back to Heathrow Airport, in a similar fashion to Ian and Barbara in the first place, although he now has a far better handle on controlling the TARDIS, they seem to get knocked off course all the time.
Then it's Peri, and Mel and Ace, all from Earth, all want to see the past and future, he is basically educating Ace for some grand plan, although we never really find out what.
Apart from the Paul McGann thing, the next time we catch up with him he's just regenerated after the end of the Time War, he thinks he's destroyed his home world, so obviously he's going to head to his adopted planet to lick his wounds, and there he meets Rose, and for the first time ever actually invites someone to travel with him. And guess where she wants to go, etc, etc, etc.
No mystery there at all.
