What does Biff do that elates Willy?
Willy has poured quite a bit of himself into Biff, emotionally. He has expectations of greatness for this particular son, yet Biff is resistant to Willy’s affections and maintains a cold distance as we meet him in the play.
We learn that Biff has caught his father cheating on his mother in a hotel room. This is the reason for the rift between father and son. It is Biff’s loyalty to his mother that leads him to announce that he will attempt to get a start in sales, selling sporting equipment, as his mother has begged him to try to please his father.
This plan is a complete turn-around for Biff and it suggests that Willy’s hopes for Biff have been justified all along. Willy is elated by two elements of this plan.
First, Biff might not hate Willy after all and, second, he is choosing to follow in his father’s footsteps and to take his advice to become a salesman.
This reinforces Willy’s conception of himself as an acceptable, respectable person raising fine boys and it allows him to feel that he is no longer being rejected by his favorite son.
During the climax of the play back at home after the scene at Frank's Chop House, Biff confronts Willy about his epiphany. Biff realizes that the perceptions he had held for his entire life (that Willy instilled into him) up till this point were false. Willy essentially says that Biff is only saying this to spite him, Biff then begins to cry for his father's delusional and forgone state. At this point, Willy replies "isn't that remarkable? Biff- He likes me!"
Happy, being the more burdensome and less loved son tries to attract attention to both of his parents attention by saying "I've lost weight" and "I'm getting married, Mom. I wanted to tell you." He's either just brushed off, or ignored completely. During the requiem, he vows to pursue his father's life as to not have Willy's death be in vain, being another example of his attempt to gain reassurance and love from a very unloving family.
At the end, Ben changes from the only one whom had deterred Willy from killing himself, into approving and convincing him that it is the only way to leave his legacy for Biff.