The Doctor grinned. "And she doesn't like just anyone either. My friend Jack, she went to the end of the universe trying to shake him off, once upon a time."
This, right there, was why the Doctor needed a companion. He needed someone to smack him around the gourd when he was being a moron. Like now.
"No, no it's fine," he said, waving her off as best as he could, not meeting her eye. "It's just...you being here might change things. Might not. Never know," he shrugged, talking a little more erratically than usual. "You can never be sure. And given that you might be able to go back someday...I...I can't tell you."