Willy's main problem is that he is extremely impractical. He thinks that an individual can advance in the world almost solely on the basis of being popular. He does not discount the ideal of hard work - in fact he himself is shown to slave away for very little reward all his life - but he regards it as less important than being sociable and personally attractive.This attitude proves to be his undoing in the hard-headed and endlessly competitive world of business.
Willy clings to his philosophy even when he himself has been cast aside in his old age by his employers, as his more sober neighbour Charley points out:
Willy, when're you gonna realise that them things don't mean anything? (Act II)
Willy not only damages his own career with such an unrealistic attitude, he impresses these notions on his sons, Biff and Happy, to such an extent that it helps to ruin them too. Meanwhile Charley's quiet, studious son Bernard, whom Willy used to mock for being 'anaemic' and 'a worm' (Act I) becomes a highly succesful lawyer, while Biff and Happy are left to flounder, thus totally disproving Willy's earlier prediction to his sons:
Bernard can get the best marks in school, y'understand, but when he gets out in the business world, y'understand, you are going to be five times ahead of him. That's why I thank God that you two are built like Adonises. Because the man who makes an appearance in the business world. the man who creates a personal interest, is the man who gets ahead. Be liked and you will never want. (Act I)
Willy means well in attempting to cultivate his own and his sons' personalities but he is misguided, and he tries to influence his sons too much. He remains enmeshed in the American Dream, the vision of vast material wealth and social success. This leads him to neglect other possibilites in life, such as his own talent as a carpenter, which could have been more financially rewarding and also more emotionally satisfying for him. Instead he pursues his unrealistic dreams of somehow making it big right up to his death, even although this causes a final fatal rift with Biff.
After Willy's suicide, Happy is shown to remain deluded, just as his father was, although Biff finally frees himself from Willy's damaging influence, choosing to strike out his own way in life.