Madge: I don't know why I keep shouting at them.
The Doctor: Because every time you see them happy you remember how sad they're going to be. And it breaks your heart. Because what's the point in them being happy now if they're going to be sad later. The answer is, of course, because they are going to be sad later.
~ The Doctor, the Widow, and the Wardrobe

Author: Steven Moffat