The Doctor would've liked to have thought a lot of mental stimuli would've gone through his head at that moment. He would've been working exactly how to show Jack he cared without having to actually confess anything, and therefore change it. He would've been plotting out exactly what to do next, exactly how to pull him back and show him, because Jack wasn't demanding words, he wanted action. And the Doctor was a man of action, generally.

Instead, the only thing he was thinking was the only thing that came out of his mouth:

"Don't I?"