It wasn't easy, leaving Jack behind on the TARDIS. It wasn't that he doubted Jack would take care of her. No, no, that wasn't even a doubt. Nor was it a worry that the TARDIS would take care of Jack. She'd warmed to him over their time together, she liked the influence he had on her Time Lord.
It was just...leaving him behind. Old and weak and so obviously needing reassurance, and the Doctor left him. He left him for the Master. The Master who, as Wilf pointed out, was probably the one who would kill him. The Doctor could smell him in the air, feel him like the quiet aura of a muted television in a quiet room.
And then heading out, finding the Master again. Having him so close, being able to feel him right next to him. Trying, and failing, to pull him to his side again. The Master needed him. He did, even if he didn't realize it. Waking up in a cold wasteland after watching the Master kidnapped, only just realizing that the drums in his head were real. But it was too late, and he was somewhere, still clinging to the Doctor's nose. Wilf's only suggestion was the book Donna had given him.
Donna. The Doctor's Donna, somewhere in there, trying to save him.
Wilf followed the Doctor without question into the TARDIS.
"You're not leaving me with her," he said, gesturing back to Sylvia, who was already furious at the Doctor's arrival."
"Fair enough." He held open the door for the older man and followed.