Alice is Bob's master. She could be a master of anything, from martial arts to cooking, anything that involves having a teacher. Bob is very thankful to Alice, since her teaching was invaluable. Perhaps he has more to be thankful for: maybe Alice actually raised Bob, or gave him shelter. Whatever the case, Bob is very thankful to Alice and would do anything for her. And Alice milks it for all its worth. Compare Wax On, Wax Off, where the chores the student is performing are actually genuine teaching tools. Trickster Mentors are very fond of this trope.