A belated evidenciary addition to my previously-stated theory that the Doctor is, in fact, a Dalek. It’s going to get spoilery in here!
The major thematic thrust of this year’s Xmas special was about how the Doctor used to be kind, but no longer is – with the implication that he stopped being kind around that time that he lost Amy and Rory. While I actually have major problems with this suggestion*, it does fit with the whole “turned into a Dalek” idea – especially since in the episode where he turned into a Dalek (yes, I’m talking about it as a certainty now; it’s just less linguistically awkward – you can mentally add ifs if you like), the emotional characteristics of Daleks were the main way in which the potential transformation was talked about.
I also feel like Oswin/Clara’s reappearance** strengthens my theory. We know she’s inhabiting the mind of a Dalek – maybe it’s the Doctor-Dalek’s mind, and he’s hallucinating her to some extent. I haven’t worked out the details of how this could be the case, but I’m sure it’ll involve wibbly-wobbly, timey-wimey hand-waving.
*I would argue that the Doctor in the reboot series has pretty much never been motivated by kindness (it’s certainly never been a primary motivation).
He’s motivated by curiosity, and his sense of justice, and beauty, and his love of other races in general and humanity in particular. He’s also motivated by opportunities to be clever. But not by kindness. Donna tried (and succeeded to some small extent) to make him value kindness for its own sake, but it’s simply not one of his major characteristics, from where I sit.
**So excited about this, by the by! I fell in love with Oswin almost instantly, and was sad when I thought she wouldn’t be a recurring character.