The Doctor had been paying far more attention to his screwdriver than to where he was walking. That was why as he quickly turned a corner he nearly collided with someone. His head shot up, ready to apologize to whoever it was he just about walked into, but that went out the door when he saw Jack.
"Romana told me you were around here," he said with a grin. Jack could be useful in a fight with Daleks. They could zap him all they wanted and he'd keep getting right back up.
Okay, well, Jack might not like that too much, as it would probably hurt anyway, getting your nervous system destroyed over and over and all that. But the Doctor wasn't always known to have the... kindest of ideas.