"We had to leave her behind. She can't be found. Especially since we're in the beginnings of dark times, and he would have used her most terribly." Caitlyn says understanding that they had had to leave the TARDIS behind and even giving his own reason as to why they had to go. She softly plays with Jack's hair as he slept knowing that he needed to build up his strength to what it had been before he had gotten sick.
"An' besides, he just got out of bein' sick." Rose added on not wanting the Doctor to feel guilty for Jack's current condition. It might have happened anyway, even if he hadn't met them again. But at least now he was around those that deeply cared for him.