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?