Throughout the play the Lomans in general cannot distinguish between reality and illusion, particularly Willy. This is a major theme and source of conflict in the play. Willy cannot see who he and his sons are. He believes that they are great men who have what it takes to be successful and beat the business world. Unfortunately, he is mistaken. In reality, Willy and sons are not, and cannot, be successful.
Certain lines in the play point to this character flaw that is present in Willy, Hap, and (for a time) Biff. For example, Willy believes that to be well liked is the means to being successful. This is an illusion that Willy lives in. Also, on the literal level, Willy very often lapses into a flashback and appears to be reliving conversations and situations that occurred years ago. This itself is an inability to see reality.
This reality versus illusion problem eventually brings about Willy's downfall. In the end, Willy believes that a man can be "worth more dead than alive." Charlie, always the voice of reality tells Willy, "A man isn't worth anything dead."
Willy is also unable to see change. He is man lost in the modern era of technology. He says, "How can they whip cheese?" and is constantly "In a race with the junkyard."
Willy has lost at trying to live the American Dream and the play can be viewed as commentary about society. Willy was a man who was worked all his life by the machinery of Democracy and Free Enterprise and was then spit mercilessly out, spent like a "piece of fruit."