In Death of a Salesman, what compels the Loman family to deceive each other?

You are right in identifying deception as a key component
of this moving and poignant play. It is important to realise, however, how success and
in particular the American Dream has entered into the consciousness of the family,
particularly Willy and Biff and Happy. The idea that if you just work hard enough you
can become successful and make money obscures the corollary: if you are not successful
it is obvious that you haven't worked hard enough and therefore you are a failure. This
is the reality that Willy spends his life trying to ignore and conceal, and it is clear
that this has rubbed off onto his sons, as they have inherited their father's propensity
to imagine grand plans and consider themselves to be better than others whereas, in
fact, the reverse is true and the plans they design are often impractical and doomed to
failure.

Of course, it is partly the inescapable reality of
Willy's failure that results in his suicide. He is unable to face his wife and sons and
above all, himself, when the knowledge of his inability to succeed in his dreams finally
and irrevocably penetrates his psyche. In spite of his sad end, it appears that Biff at
least has learnt his lesson, realising that his father "didn't know who he was." Happy,
however, seems to find even greater impetus from his father's death to continue the
pursuit of the ever-elusive American Dream:

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I'm not licked that easily. I'm staying right in
this city, and I'm gonna beat this racket! The Loman
Brothers!

It appears that the
capacity to dream and deceive is still something that is alive and strong in Happy, even
after the failure of his father's life.