When Jack and Phoebe are having lunch and Jack does not want to say grace, he notices that Phoebe's eyes grow large with alarm. The narrator says, "and, though he did not know why, his heart gave a jump." Why does Jack's heart jump when he sees how fright

Jack realizes Phoebe is upset and alarmed and he does not want her to feel this way. For a moment, he comes in touch with his love for her and his need to protect her, and quickly tells her he has already said grace. He does not know why this matters to Phoebe, only that he does not want to cause her any pain or sadness.