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.