RORY: But I don't understand. Why am I here?

DOCTOR: Because you are. The universe is big. It's vast and complicated and ridiculous, and sometimes, very rarely, impossible things just happen and we call them miracles, and that's the theory. Nine hundred years, never seen one yet, but this would do me. Now get upstairs. She's Amy and she's surrounded by Romans. I'm not sure history can take it.