Jack moved to crouch in front of Susan, a little concerned by her reaction. And a little wary of flirting quite as blatently with her if the Doctor was her grandfather. He reached for her hands, rubbing his thumbs over the backs of them, trying to provide at least a bit of comfort.
"Because they're crafty bastards with two things they do well," he replied, though he doubted she was really expecting an answer. "Survive and kill."