Willy thought of himself as a man who could succeed by simply "being liked." In truth, his arrogance of thinking that he was liked did not help him see that he was, in fact, not that very well liked. He talked down to people in his heyday, he held a mistress, and he would make himself look bigger than what he was. He passed down on Biff the same gospel. Biff, in turn, lived it. He went into the football team, and saw his Dad as his idol...until he found out his Dad was having an affair.
After that moment, Biff became lost, and was not doing anymore anything his Dad expected him to do. He was no longer a football player, nor a good student, nor a loving son. Willy, who saw himself cathartically through Biff, thought that Biff was a massive underachiever with lots of potential. However, how could Biff have been motivated to pursue the dreams his father had for him when his father had fallen violently out of the throne Biff had built for Willy?
Biff's only choice was to stay away to try to find his true self. Albeit, Willy continues to vicariously live through Biff in his memories. We know, from those same memories, that Willy really saw Biff as an extension of his imaginary self. When Biff went away, Willy felt that Biff was not living up to the supposed potential that he had as a result of being the son of Willy (in his successful alter ego). What this tells us about Willy is that he really saw himself under a higher light than he really was. It also shows that he expected Biff to follow suit, and that Biff's turnaround was a symbol of Biff's weakness in Willy's mind. In fact,Biff's reaction may actually have been a symbol of Biff's inner strength in finding himself away from his father, and in coming back to the reality that Willy, as well as himself (thanks to Willy) had placed themselves in pedestals to which they do not belong.