The theme of human isolation is juxtaposed to "the nuclear family setting" in Arthur Miller's Death of a Salesman because, although the entire family is together throughout the whole play, each individual member of the Loman family is battling a very lonely and personal struggle.
The first battle that we witness is Willy's own. He is a man whose sense of personal inadequacy prompts him to imagine himself as a man who is vital to his company, attractive, and successful. However, Willy opens up to his self-deceit when he discloses in several occasions that he does not feel up to pair with his peers. His continuous attempts to prove his worth only leave him empty-handed. His last resort is to vicariously live through the successes of his eldest son, Biff, who happens to be attractive, and a leader in sports, during his high school years. Yet, when father and son break their mutual bond, and Biff does not accomplish what Willy hopes that Biff would, Willy is left alone "with himself". As a result, Willy lives his latter years in denial of his lack of everything and battles his personal demons. These manifest in the flashbacks and hallucinations that he experiences up to his very last day of life.
Similarly, Biff and Happy are alone even if they are in physical proximity. Happy, who is (as his name implies) a "happy-go-lucky" character, lives his life superficially. He does not attempt to find himself and grow up. Instead, he chases after women who are already engaged, he lives in his own apartment, but he confesses that he does not feel satisfied. This is because Happy has learned to live life with the foolish idea that things are not what they are, but what they "appear" to be. Hence, the mere appearance of things being OK is what leads him to follow his father's lead to live a life of pretense and lies.
Biff has figured out the main problem of the family and tries, without success, to expose Willy's bad influence in everybody's life. However, both Happy and Linda are too used to Willy's ways of making everybody believe that their lives are successful even when they are, obviously, not. Similarly to Happy, Biff feels dissatisfied. However, Biff's real problem is that he has found what makes him truly happy but, in fear of disappointing Willy, he refuses to accept it. In the end we find that Biff is probably the only member of the Loman family that is willing to reject Willy's ways.
Linda is probably Willy's biggest "victim." As a typical enabler, she encourages Willy's fantasies of grandiosity while pretending that his flaws are mere trifles. She does not even confront Willy when she realizes that he has tried to kill himself. Her loneliness is remarkable, as she even confesses that she goes "day by day" in her daily battles to try to soothe Willy. Since Linda has no other support system but Willy, himself, her life is basically a stage where she can act happy just for Willy. She realizes during Willy's funeral that they have all lived an isolated life of mere make-believe, all along.
Therefore, we can see that the Lomans have completely fooled themselves by following Willy's lead. Willy's desperate attempts to make his life make sense results in his influencing everybody to pretend that his fantasies are actually real.