Moffat: "The series has always been the story of how the companion changes, not how the Doctor changes. The Doctor doesn't change very much. That's always the story."
Sepinwall: "So the childhood meeting is just an easy way to illustrate that, rather than revisiting a former companion years later?"
Moffat: "I like things that force the Doctor to address that he's aging much more slowly than everyone else. I think that's interesting, whether you do it in the simple, cartoony way of him missing an entire growing up, or just seeing Amy and Rory. They're getting married, getting a house, while the Doctor is remaining fundamentally the same, while they grow up around him. Which is why he tries to get out of their lives. It's too hard. "
Sepinwall: "So the childhood meeting is just an easy way to illustrate that, rather than revisiting a former companion years later?"
Moffat: "I like things that force the Doctor to address that he's aging much more slowly than everyone else. I think that's interesting, whether you do it in the simple, cartoony way of him missing an entire growing up, or just seeing Amy and Rory. They're getting married, getting a house, while the Doctor is remaining fundamentally the same, while they grow up around him. Which is why he tries to get out of their lives. It's too hard. "
The full interview is here: Interview: Doctor Who showrunner Steven Moffat on Amy, River, Rory and more - HitFix.com: "