It struck me as an epiphany story.
Even all of the Doctor's guilt and self loathing and horror weren't enough to defeat the emotion eating space monster. All of his negative emotions weren't enough but the positive ones of Clara were.
We've seen a gradual shift in the Doctor from the worn down, cynical, secretive, scarred survivor that we first saw signs of as far back as the Sixth Doctor (yes six. I am that old) and really highlighted with the ninth and Tenth, to a Doctor that is finally letting himself live and let go of his pains.
Usually when this kind of thing happens to a character, he dies soon after. But, at least he dies content.
Hopefully, the WHO writing team isn't thinking along those lines quite yet. I happen to like Matt Smith.