The conflict between Willy and Biff seems to have developed in the wake of Biff's discovery (when he was very young) that Willy was having an affair. But more fundamentally, the tension between the two is owing to Willy's unrealistically high expectations for his son, and Biff's inability to meet them. Even as he has tried to hide his failures from his son, Willy remains convinced that Biff has failed to achieve success in life just to spite him, and he also believes that Biff has no respect for him, regarding him as a "nobody." Biff, on the other hand, blames his failures on his father, claiming that he filled him "full of hot air" and that trying to be a successful businessman, as his father wishes, is just not for him:
Pop, I’m nothing! I’m nothing, Pop. Can’t you understand that? There’s no spite in it any more. I’m just what I am, that’s all…. Will you let me go, for Christ’s sake? Will you take that phony dream and burn it before something happens?
This outpouring of emotion in the final act, ironically, convinces Willy to commit suicide, believing that his son will idolize him because, through his death, he will provide him with life insurance money.
the confllict between them mainly happend because willy has expect a lot from Biff, so when Biff did not achieve the dreams that his father made for him, Willy has been angry and deceived. Willy has always lament himself of not being a good father because of this.
Another thing is that when Biff came to see his father at Boston, he found his father with another women. In other words, he found Willy cheating his mother and Biff was very affected and his relation with his father completely changed. Earlier in the play we fouund that Biff and Wiilly was having a good relationship together, in fact he was paing more attention to biff that happy. but however this realtion has changed because Biff has came to Boston and found his father with another women.
Biff who has always been listening to his father completely change.
Willy: (grabbing for Biff) I gave you an order!
Biff: Don't touch me you - liar!
Willy: Apologize for that!
Biff: You fake! You phony little fake! You fake!
This is how the relation of the tension has been developing and how it's started.