This is not our Doctor. Not our Matt Smith.
This sullen, withdrawn, fatigued Doctor is not the same man who cheerfully battled Daleks and rode a triceratops like a pony.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.
Apparently there was a secret plan to include Christopher Eccleston’s Ninth Doctor in DOCTOR WHO’s 50th anniversary story, but the actor has chosen not to participate after all — perhaps due to his commitments to filming Thor: The Dark World; perhaps for other reasons.
This news is certainly disappointing, but Eccleston has a right to make the call. What I find most intriguing is that a number of DOCTOR WHO-related websites — including Kasterborous.com — have been sitting on the news that Eccleston apparently had agreed to appear “in principle” at some earlier junction. I tip my hat to those websites for keeping the surprise under their hats, leaving open the possibility of genuine fan surprise when the show was broadcast. Good on them, for valuing story integrity over a momentary “scoop.”