It's not that Martha begrudges that Doctor having some fun. She just would have preferred that she not be the victim of his little prank, although her heartbeat is already normalizing, so she'll forgive him. This time.

She's a little surprised that the Doctor doesn't have some story about an adventure with a ghost, honestly, but she shrugs her shoulders at that question. He makes a point -- it still doesn't give him license to sneak up on her like that.

"You'd do good to find yourself some chains, then," she says, and while her tone is firm she mainly means it as a joke. If he really wants to go spook other people, she won't stop him. In fact, her expression becomes more somber as she leans toward him and looks him over. As comical as it might be now, the Doctor still died.

"I'm... I should have been there, to help you. I'm sorry."