Amy: You are a bloke and you don’t know it. [puts her arms around him] And here I am to help.
The Doctor: [pushing her away] That is not why you’re here.
Amy: Then why am I here?
The Doctor: Because! [lowers his voice] Because I can’t see it anymore.
Amy: See what?
The Doctor: I’m 907. After a while… you just can’t see it!
Amy: See what?
The Doctor: Everything! I look at a star and it’s just a big ball of burning gas and I know how it began and I know how it ends and I was probably there both times. After a while, everything is just stuff! That’s the problem. You make all of space and time your backyard and what do you have? A backyard. But you, you can see it. And when you see it, I see it. 

(Thanks to goodreads.com for the quote!)