This sullen, withdrawn, fatigued Doctor is not the same man who cheerfully battled Daleks and rode a triceratops like a pony. This Doctor is grieving. This Doctor is pain, and he cannot heal himself. He is a man in crisis. He is a man in hiding. Since when does the Doctor hide?
The morose shift in personality is due to the loss of Amy — and Rory, but… Amy — in the last story, when the Weeping Angels sent them into the past to live themselves to death. Now the Doctor has given in to his pain and doesn’t want to want watch any more companions die — by violence or by simply outliving them. He hates endings. He’s literally hiding from his past in London’s past while nursing two broken hearts.