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:
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.