This Doctor, unlike many others before him, was given a warning as to when his current life would end. After bidding farewell to Carmen at the end of Planet of the Dead, she told him that his song would be ending and that something would be returning from the dark, which understandably scared the Doctor. He had already avoided death once and did everything in his power to stop it from occurring, which included him electrocuting some poor sap just to keep him from knocking that fateful fourth time. When the Time Lords circumvented the time lock of the War and brought Gallifrey to Earth, the Doctor knew that the prophecy was coming true. Even though things were ultimately righted, Wilfred Mott then knocked 4 times to be let out of a radiation chamber he was trapped in. For a fleeting moment, viewers could see hesitation in the Doctor's eyes, as if he was considering leaving him in there to save his own skin. He ended up making the right choice, of course, but that moment's hesitation is still something not seen from other Doctors who have readily risked themselves in order to save their companions.