He can't let go of the idea that popularity and wealth are what are most important in a man.
In the second act, Willy receives a terrible blow. He explains to his boss, Howard, how he met a salesman when he was about 19, and admired the man's skills, and decided that sales was the very best job a man could have. But he tells Howard he's tired, and he wants to work in the store instead of on the road. Howard keeps telling him there's no opening for floor sales, and then finally tells him the truth: the company is going to let Willy go. Howard says:
HOWARD: I think you need a good long rest, Willy... And then when you feel better, come back, and we'll see if we can work something out.
He tells Willy that this is no time for false pride and that he should ask his sons for help, but Willy has run on false pride all his life. He can't let go of it now that his career is crumbling around him. In the middle of this desperation, he flashes back to his Uncle Ben, who really could close a big deal with just a phone call, who went to Alaska with nothing and came back rich. This just helps prop up Willy's notion that it should be easy to achieve wealth, making him all the more a failure because he has worked so hard but not accomplished it.
Everywhere Willy turns there are signs that he held on to false beliefs. He runs into Bernard who is grown, has two children, and is doing very well professionally. Biff, it turns out, failed math, didn't make up the credit, and never got his high school diploma. Near the end of the play, we find out why. Biff caught his dad with another woman while Willy was out of town. Biff idolized his father, but sees him give stockings to a strange woman while Linda repeatedly mends hers. More than anything, Willy has always wanted Biff to love him and to do well. As Willy's career is falling apart, he sees Biff as a failure, and all he and Biff do is argue.
Planting vegetables in the back yard that will never grow, Willy imagines his funeral large funeral with people coming from hundreds of miles to mourn his passing, and money to provide for Linda. The one thing he has done well is pay his life insurance.
WILLY:... They'll come from Maine, Massachusetts, Vermont, New Hampshire!... that boy will be thunderstruck, because he never realized - I am known!... he'll see it with his eyes once and for all... He's in for a shock, that boy!
But Biff does not think this way. He is tired of all the lies, of having Willy tell people Biff was a salesman when he worked in the stock room, that Happy is "near the top" when Biff knows he's several notches down from that on the company ladder. People have made excuses about his petty thefts for years, and now he reveals the whole truth: he spent three months in jail for theft. He doesn't want what Willy wants. He wants the free life of a ranch hand. He doesn't have Willy's ambitions.
Biff has stripped Willy of all his rationalizations, his family goes to bed, and Willy imagines one more time that he's talking to Ben. Ben says,
BEN: The jungle is dark but full of diamonds, Willy.
Willy gets in his car, speeds off, and kills himself in a crash.
Charley says at the end of the play that "a salesman has got to dream... It comes with the territory." Willy's tragic flaw was that he imagined what he wanted to accomplish and then lived as if those dreams were reality. He saw only one way to succeed. He wasn't suited for that path but could not let go of the illusion. Willy was never a famous man. He never made a lot of money or achieved great things, but the real tragedy in his life was that he was too proud to allow himself to be satisfied with who he…