Here's why:
* Willy's distorted reality: Willy lives in a world of fantasy and self-deception. He believes he's a successful salesman, a respected man, and a good father. However, Biff sees through this façade and recognizes his father's failures.
* Biff's disillusionment: Biff has discovered the truth about his father's lies and his own shortcomings. He feels betrayed by Willy's constant fabrications and his own inability to live up to his father's expectations.
* The confrontation: Biff finally confronts Willy with the truth about his life and his own disillusionment. He tells Willy that he is not a successful salesman, that he doesn't respect him, and that he feels trapped by his father's expectations.
It's important to remember that Biff's actions stem from his own pain and frustration, and he is trying to break free from Willy's distorted reality. While his honesty may hurt Willy, it ultimately forces him to face the truth about himself and his life.