I would only add two things to the first post which covers it pretty well. Willie has always needed Biff to succeed, particularly after the Boston incident, because Willie thinks that Biff is falling apart because of Willie's "affair." He needs Biff to succeed in spit of what he has learned about his father, almost as an act of forgiveness. Biff's learned that his Dad is human; this is a difficult thing for any son to learn about his father. However Biff had failed because he believed the same dream that Willie believed ... that the cult of personality was what really counted in this world, not performance; that he would be passed in algebra rather than pass it himself.
I also don't know that Willie consciously decides that he can no longer live a life of illusion; I'm not sure that Willie ever achieved that level of self-awareness. He had been thinking of/trying to kill himself over time because he was a failure, and he had learned the horrible lesson that a man IS a piece of fruit; that you can eat the fruit and throw away the peel. Who knows if Willie was ever a good salesmen; the evidence seems to indicate that he was not. And perhaps more important than work, he had failed his son because Biff was never the same after Boston. It's not far from realizing that you're a peel to a total sense of hopelessness.
Willy does not understand why Biff has not tried to be "successful" and he is living in his own fantasy world, so Willy was not going to be capable of ever truly understanding why Biff was a "failure." Biff, in my opinion, was never a success not because of the expectations that his father had for him, but because he simply didn't have it in him to be successful; unlike the last response, I do not believe Biff would ever succeed. He was rather lazy, in my opinion, and never really tried to be successful or motivated. Even though he realized the dream world Willy was living in, Biff never really tried to help his father. He just let things be.
Willy thinks Biff fails so often simply to spite or anger Willy. However, Biff is much more realistic than his father and eventually rejects his father's life, which is based on illusion. Realizing that he cannot gloss over some mistakes of his past, Biff answers his father by saying, "I'm no good, can't you see what I am?" Unfortunately, all Willy can see is the old Biff, a high school football star, and not a young man desperately trying to sort out his life. In the end, Willy can no longer live a life based simply on illusion and kills himself. However, Biff's rejection of his father's philosophy suggests that Biff will probably become a productive citizen who can live in a realistic world.
I understand what the teacher's were saying earlier but I don't think that's why Willy killed himself. He was so lost in his own make believe world that he believed killing himself was his last move to be successful. He would have all the buyers there, and then biff would see what a great man he was. how well liked he was. Willy gives his reason as to why he killed himself. Unfortunately, we aren't answering the question asked of us by discussing Willy's suicide. Willy thinks that Biff has all he needs to be a great buisiness man. Unfortunately, Biff has no skill for buisiness because he didn't train for that. he was groomed to be a pretty face with a smooth voice and nothing more. Willy wants to live vicariously through his son. Biff wishes to be free from the constraints of the buisiness world, to work under the open sky and do work that has a visible and tangible point. Biff fails at buisiness because it is not where he wants to be.