In Arthur Miller's Death of a Salesman, Willy is a great deal to blame for his sons' lack of success because of his unrealistic view of the world, and his propensity to live in a world of fantasy (seen in the theme, appearances vs. reality).
Willy Loman lives in the past, forever referring to his success in the business world. And while he may have had some success as a younger man, he lives a life surrounded by the ghost of his brother Ben—a great success in his words:
Why boys, when I was seventeen I walked into the jungle, and when I was twenty-one I walked out...And by God I was rich.
(We have no way of knowing if this is true. We know Ben is dead, but not under what circumstances, or if he was actually wealthy.)
Willy always defends his actions, makes promises of impossible future successes and laments a life of missed opportunities. He fails to give his sons the example of a strong father. He sometimes makes excuses for the boys, or—as with Biff—finds fault for a lack of success.
While Willy lives in the past, he also ignores memories of things he has done that have negatively impacted at least one of his sons—Biff.
Biff was his high school football team's star quarterback. He had been awarded a scholarship to college, but he failed senior math. His friend Bernard recalls that Biff planned to take the class in summer school; and then to go away to school—until he visited Willy in Boston, who was there on a business trip. Willy, who may have seemed a hero to Biff before, destroys Biff's image of his dad, as well as his son's desire to do anything with his life. It was as if Biff's life was stopped dead in its tracks. Bernard recalls when Biff changed:
Willy, I remember it was June, and our grades came out. And he'd flunked math...Biff just got very angry, I remember, and he was ready to enroll in summer school...He wasn't beaten by it at all. But then, Willy, he disappeared from the block for almost a month. And I got the idea that he'd gone up to New England to see you...And he came back...and took his sneakers—remember those sneakers with "University of Virginia" printed on them?...took them down in the cellar and burned them up in the furnace. We had a fist fight. It lasted at least half an hour. Just the two of us, punching each other...crying right through it.
While Willy refuses to face the significance of that time in Biff's life, the audience knows that Biff showed up in Boston, surprising his father—who was with a woman (not his wife, Linda), in the midst of an affair. Biff never told his mother, but he was never the same with Willy.
Happy (Biff's brother) also lives a life based on lies. He is not interested in seriously working, and has a reputation as a womanizer (much like the hidden life Willy led in the past).
For instance, Happy is at the bar and sees a girl enter. Right away he begins to play his "pick up" game:
Why don't you bring her—excuse me, miss, do you mind? I sell champagne, and I'd like you to try my brand. Bring her a champagne, Stanley.
Happy does this not only to impress the waiter (Stanley), who thinks Happy is a real "operator," but also to impress the young lady. Happy is not in champagne sales. Happy has no drive, and no desire to work hard and make something of himself. His father, too, "talked the talk" for years. Happy is as lost as his father is.
Under other circumstances, the boys could have been stronger, but Willy never shows them how.