Because of Willy's own insecurities and uncertainties, he favored one son over another. Biff was his star, his dream. He poured all of his love and hope for the future and belief in success into the young Biff. Biff, the football hero, Biff the Adonis, Biff the popular, Biff the leader.
Happy, Biff's brother, was relegated to a far-off second place. He tried to get his father's attention, but he could never measure up to Biff, the shinig star. Can we blame Biff for riding high on his father's love and adulation? Should he have said, Please Pop, enough about me; pay more attention to Hap? Would that have been possible; would any boy be able to be so egalitarian, so self effacing? Especially one so pumped up with years of almost exclusive support from his father?
So, when Biff finds out the truth about his father in that hotel room in Boston, when he finds out that his father is a liar and a cheat, the real question is: is it a complete shock to him that this salesman who is is father was just full of hot air? Or did he secretly know all along that poor Willy needed him as a prop for his exhausting life and for his gnawing failures? Could he have known this; would he, years ago, have been able to see the truth about his father and admitted it to himself?
This is one of the great mysteries of the play. But whatever the answer is, this is clear: Biff and his father were a doomed pair riding high on a "shoeshine and a smile," and when Willy was found out, when Biff stopped smiling back, they both came crashing down.
There is also something to be said for Willy's statement that Biff's failure has all been "for spite." When Biff discovered that the dad he idolized was living a lie, he did, in some ways choose to fail. If some of what Willy had said was untrue, then it all must be--so why should Biff bother? In the end, it is indeed Biff's fault. A stronger person could have chosen to be better than what his dad revealed himself to be, but for Biff it was easier to give up, as he had a convenient person to blame.
In the final analysis, one of the lasting legacies of Miller's work is the idea that human beings are the sum total of their actions and beliefs. One of the truths spoken in the play comes out of Biff's mouth: "I'm just what I am, that's all." In this line lies much in the way of assessing blame or responsibility. On one hand, Biff's life is what it is, as he is the ultimate agent of action for his existence. His addiction and problems with stealing, and the disillusionment present are within his control. There is an undeniable level of responsibility borne by his father in not being able to guide him correctly in changing his ways as a youth and not instilling the values appropriate to finding some level of happiness. However, in the truth of self awareness and understanding lies an aspect of personal responsibility and acceptance. Within this, one cannot lie all of Biff's predicament at Wily's feet. There are some elements for which Wily must assume responsibility and some that are not his doing, lying at Biff's doorstep of moral and ethical responsibility.