Miller's own words might be a helpful guide in answering this question. Consider his own thoughts in an interview given about an audience's reaction to his play:
[Audience members] were weeping because the central matrix of this play is ... what most people are up against in their lives.... they were seeing themselves, not because Willy is a salesman, but the situation in which he stood and to which he was reacting, and which was reacting against him, was probably the central situation of contemporary civilization. It is that we are struggling with forces that are far greater than we can handle, with no equipment to make anything mean anything.
From the above idea, Miller seems to be asserting that there is a fatal combination of obstacles from social context and personal desire to assimilate to such a context. The materialization of the American Dream is a vision in which there are many more casualties than success stories. Willy's primary set of obstacles resides in trying to achieve this vision in a narrow and ultimatum- like context. The real genius of the drama is that Miller does not make it out to be simply the social setting's fault, nor does he unfairly condemn his own protagonist. Rather, Miller is able to draw out the idea that this "struggling" is challenging when social obstacles and personal obstacles collude to prevent individuals from being able to embrace a vision of happiness or contentment within consciousness. This lack of emotional grounding helps to create the material and social obstacles that end up denying Willy any hope of happiness.