The Return of Doctor Mysterio - S10-E1
The Doctor: [Looking at the Superman comic] Take a look at that picture.
Young Grant: OK.
The Doctor: Now, this one. Take a good, long look. Takes a moment to see it.
Young Grant: See what?
The Doctor: Superman and Clark Kent are one and the same person.
Young Grant: Are you serious?
The Doctor: Yeah. Look, I drew specs on Superman.
Young Grant: Everyone knows they're the same person.
The Doctor: Uh. Well, Lois Lane doesn't. And she's a reporter.
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 after everything the Master's put him through hurts him more deeply than death would.
Captain Defenestrator