Final answer:
Lucy believes that success is extremely difficult to understand and can change people, as seen with Isabella, who loved acting for its own sake but is now caught up in the pursuit of fame.
Step-by-step explanation:
Lucy finds success to be extremely difficult to explain. She observes that Isabella, who initially had no interest in fame but simply loved acting, is now swept by the changes that success brings. Success can often alter people's priorities and values, making them forget their original passion, as in Isabella's case, where being happiest in playing a part shifted perhaps towards the trappings of fame.