What really made the Doctor want to take Amy with him?

Question

Warning, there's a few spoilers for Series 4, 5 and 6!!

In the 2009 episode "Planet of the Dead", Lady Christina asked the Doctor to take her to see the stars. The Doctor refuses, and says:

DOCTOR: People have travelled with me and I've lost them. Lost them all. Never again.

But still, after a fairly short time he meets Amelia Pond, and doesn't even hesitate before he takes Amy with him.

We get a few explanations about why he does that.

In the Series 5 finale "The Big Bang", just before the Doctor flies the Pandorica to the TARDIS:

DOCTOR: There was a crack in time in the wall of your bedroom, and it's been eating away at your life for a long time now. Amy Pond, all alone. The girl who didn't make sense. How could I resist?

And, in the Series 6 episode "The God Complex", when the Doctor is saving Amy from the minotaur:

DOCTOR: Forget your faith in me. I took you with me because I was vain. Because I wanted to be adored. Look at you. Glorious Pond, the girl who waited for me. I'm not a hero. I really am just a mad man in a box. And it's time we saw each other as we really are.

So, because of rule 1 (The Doctor lies), and because I got the impression from the tenth Doctor that he’d never take a companion again, I'm asking whether there are any other motives that made him take Amy with him?

Answer 1

I think there are two possible reasons for this apparent contradiction.

The Doctor changed so much that he reconsidered his decision.

When he met Lady Christina, he was close to the end of his tenth incarnation; he'd lost several Companions, and the loss of Donna was evidently still fresh in his memory. In that frame of mind, it made sense for him to refuse to take on any more Companions; he was too full of regret.

But after his regeneration, he became a brand new man: not just a new face, but an entirely new personality, as always happens upon regeneration. He became more carefree and flippant, so it makes sense that he'd be willing to take on a new Companion simply because "what the hell".

The difference between the Tenth and Eleventh Doctors is summed up nicely by the Moment:

MOMENT: The man who regrets and the man who forgets.

_{-- 50th anniversary special, Day of the Doctor}

Ten refuses a Companion because he regrets his past losses; Eleven couldn't give a damn.

Amy was so special that he had to take her on.

^{Warning: unrestrained opinions may follow!}

Lady Christina would have been an amazing Companion for the Doctor: she complemented him so well, and they made a perfect team together. She was skilled, sassy, and they'd already proved how well they could cooperate. She was one of the greatest Companions that never was ... but it wasn't necessary for him to take her on. If he'd wanted a Companion, she would have been perfect; but he didn't, and they could both survive separately.

Amy, on the other hand, was more than just a Companion: she was special. She was the most important woman in the universe because she had a crack in her bedroom w... zzzzz ... where was I? Oh yes, one of the "mysterious and oh-so-important women" of the Smith era. The trademark of the Impossible Girl was taken by someone else later, but it would have fit Amy pretty well too. She spent her entire childhood next to a "crack in the universe" that should have swallowed her up, and in the same building as Prisoner Zero, but somehow survived everything. The Doctor needed to solve this mystery. Whether he wanted a Companion or not, he had to take her with him so that he could unravel the mystery of her existence.

DOCTOR: And you asked me why I was taking you and I told you there wasn't a reason. I was lying.
AMY: What, so you did have a reason?
DOCTOR: Your house.
AMY: My house.
DOCTOR: It was too big. Too many empty rooms. Does it ever bother you, Amy, that your life doesn't make any sense?

_{-- Series 5 Episode 12, The Pandorica Opens}