The Doctor blinked...he hadn't actually realised it. Not until Brax mentioned it, anyway, and then it made sense. He wasn't feeling worthy of...well, being himself, because the Master didn't think he was. Or maybe the Master didn't think he was worthy because his own self-worth was so low in the first place.

I'm not here to fulfill your deathwish, Doctor.

He really had meant it when he said he didn't have one, though. He didn't want to die, he just wanted the Master to think he was worth the effort.

"I could do with some brandy myself, if you've got some," he replied, looking a bit sheepish.