The answer is B: Because people become prideful and believe that she will always reward them.
Fortune spins her weel, and bring down those that are at the top to the bottom, which reminds the mutability of life itself. The Monk is a corrupt religious character but he still wants to use his tales to teach some lessons; he tells stories about tragedies that happened to famous people that were in high positions, and Fortune saw to it that their pride made her depart, which caused their fall.