In Death of a Salesman, why is Willy annoyed with Biff?
Willie thinks Biff is wasting his gifts, his personal attractiveness. Wille based his career on the concept of attractiveness believing that it would be the key to his success as a salesman. Willie excused all types of destructive behavior (stealing a football, for example) because of his personal attractiveness. The foil to Biff is Bernard. Bernard represents the work ethic; he tries to get Biff to study so that he will graduate and be able to accept an athletic scholarship to the University of Virginia, but Willie mocks Bernard, saying that he will never be successful becuase he lacks this personal attractiveness. Sadly, as with many things, Willie has this all wrong.
The other possibility is that Willie is guilty because he knows that Biff's downhill slide began when he found Willie in a compromising situation when he travelled to Boston to find him and ask for his help. It is as though if Biff were successful, this incident would be wiped off the books as it were and return Willie to his one success in life --- as a father.