The Doctor has entered one of those moods. The ailing TARDIS must be the cause, but Jo's the target.
"Failed science, and all my attempts to teach you martial arts have fallen on deaf shoulders," he berates her. "Miss Grant, why do I ever keep you around?"
"Because, Doctor," she says calmly, "of the thing I've got that you haven't."
"And what is that, Miss Grant?" He peers down his nose at her.
"Social skills." She looks right back at him, dazzling smile turned to full blast.
"Quite right," he mutters, and returns to tinkering.