10 Doctor Who Questions That Always Confused You
8. Wait, Why Can't He Go Back And Save Amy And Rory?
The ending of The Angels Take Manhattan, in which the Doctor states that he's unable to return to New York to bring back Amy and Rory led to a lot of confusion and "Couldn't they just meet up in a different city?" on social media. So much so, that Steven Moffat wrote a lengthy response in Doctor Who Magazine.
...there is so much scar tissue, and the number of paradoxes that have already been inflicted on that nexus of timelines, that it will rip apart if you try to do one more thing. He has to leave it alone.
There's also a more simple, emotional explanation too. The whole of Series 7A is about Amy and Rory trying to balance normal married life with adventures in time and space, increasingly aware that one day they'll have to stop travelling in the TARDIS. That decision is eventually taken out of their hands when a Weeping Angel zaps Rory back in time and Amy decides to willingly sacrifice herself to be reunited with her husband.
The Doctor can't go back because he knows Amy doesn't want him to, she wants life with Rory. It's a perfect ending. Sadly, Doctor Who fans are so focused on the intricacies of time paradoxes that they often miss out on things like human emotion.