In Death of a Salesman by Arthur Miller, why is Willy annoyed by Biff? What does this tell us about Willy?

It is
extremely common for American parents to want their children to be successful. Part of the
American Dream is for people to have children who are better than their parents--better
educated, better looking, more intelligent, and especially better off financially. Bernard
symbolizes all that. He has wealthy, important friends. He plays tennis on their private courts.
He is going to argue a case before the U.S. Supreme Court! Willy has begun to realize that he is
never going to amount to much. Like a lot of other fathers, he consoles himself that at least he
has been biologically successful. He has fathered two strong and healthy sons and worked hard to
raise them to manhood. They could salvage his American Dream--but only if they are successful
themselves. Willy has pinned his hopes on Biff, since Biff seemed to show the most potential. He
has tried to condition Biff to be ambitious and enterprising. And Biff has tried to live up to
his father's expectations. But he can't make it....