Willy Loman, like many others after the Great Depression, bought into the New Deal and the American Dream that said anyone who works well will prosper. What Willy fails to see was the role of ability and talent in success. Willy Loman refuses to see the reality of his salesmanship, and his failure to make the sales he thinks he will make if he just keeps doing the same thing.
There's a cliche that says something to the effect that if you keep doing what you've been doing, you'll continue to get the same result. Willy, rather than evaluate why his sales were not enough to make him "top dog," simply tries harder to sell his wares. Even when his employer cuts his salary and makes him a commission only salesman, Willy fails to take the hint.
Happy follows in his father's idealistic (and delusional) footsteps, saying, "I'm gonna run that department before the year is up." There is no strategy, just a desire to be on top.
Biff, on the other hand, recognizes that he will never achieve the kind of success Willy expects. It is one of the primary sources of domestic conflict when Biff's reality collides with Willy's delusion. The final incident that led to Willy's suicide was Biff's honest assessment:
I am not a leader of men, Willy, and neither are you. You were never anything but a hard-working drummer who landed in the ash can like all the rest of them! ...Do you gather my meaning? I'm not bringing home any prizes any more, and you're going to stop waiting for me to bring them home!
While there is a hope called "The American Dream," it is not an ideal devoid of talent. Working hard at the wrong thing is as much a recipe for failure as is not working at all. Willy Loman nevers understood that lesson.