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Real World Answer: The head writer was changing from Russel T. Davies to Steven Moffet and David Tennant wanted to give Moffet as clean a slate as possible, so decided it was time to go. Who World Answer: In "Planet of the Dead" and "Planet of the Ood", the Doctor was told he will hear four knocks, then his song will end. He tried to fight it, but in "The Waters of Mars", he realized he can't, after what happened to Adelaide. In "The End of Time", the four knocks occurred a lot, until he heard the fatal knocks from Wilfred. He sacrificed his time as the Tenth Doctor to save Wilf; he didn't want to go, but he had to go.