Willy Loman's misguided interpretation of the so-called "American Dream" drives him to be consumed by an all-encompassing notion that material wealth and social acceptance are the only true measures of success in a man's life.
For examples of how Willy's poisoned view of success taints the fabled "American Dream," look to Act II when Willy's flashback recounts the afternoon before the big football game at Ebbet's Field. Charley jokes that it is just a game, mockingly forgetting the name of Red Grange and suggesting that the stadium had been demolished in order to reiterate his point. Willy, however, is so obsessed with his son's upcoming performance "in front of all the clients" that explodes at his friend for diminishing what will no doubt be Biff's moment in the sun.
Throughout the text, there is a recurrent theme of outer versus inner, where Willy projects an "American Dream" of an idealized life that never actually happened. While his real life is a humdrum nightmare of overdue bills, whipped cheese, and broken timing belts, Willy's "American Dream" is a litany of coulda', shoulda', woulda's, all of which are typically characterized by his memories (or imaginings) of great monetary wealth (as he recounts his legendary sales figures to Howard in Act II, and the lost opportunity to strike it rich in Alaska with Ben), high social standing ("I'll go to Boston... I'm well liked in Boston") , and lofty recognition (note how Willy swears that he actually gave Howard his name) that he never quite managed to attain.
This is an outstanding book, and it is well worth a read!