The Doctor: Oh, Doctor you're so handsome. Yes I am, thank you.
The Sontaran Stratagem (1) - S4-E4
Doctor: Name?
Skarl: General Skarl of the tenth Sontaran battle fleet. Skarl the undefeated!
Doctor: Oh that's no good, what if you get defeated? Skarl the not-quite-so-undefeated-any-more-but-never-mind?
Donna: He saves planets, rescues civilizatons, defeats terrible creatures. And runs a lot. Seriously, there's an AMAZING amount of running involved!
Chosen answer: The Master knows that deep down, he deserves death for the crimes that he's committed throughout his life, and since he regards The Doctor as his arch-foe, he expects it to be at his hands. The fact that The Doctor is still willing to forgive him for all of his crimes hurts him more deeply than death would.
Captain Defenestrator