Linda Loman is a woman whose dedication and love for Willy knows no boundaries. As a result, she is willing to overlook and deny anything that would come in the way of Willy's fragile ego. She lives to support and admire him. She is the ultimate wife.
We find out in the very beginning of the play that Willy crashes his car consistently. He tells his wife that this is merely distraction. Linda, however, knows that Willy is suicidal and that he has tried to kill himself in more than one way. However, she tells Willy that his tiredness and his distraction are a product of his being 63 years old and still having to drive long distances to earn a mere commission.
Willy does not want to hear this. This is when he says his famous phrase
I'm vital in New England
However, Linda knows the truth. She knows that Willy has been lowered in his company, that he is without a salary, that he is frustrated, and that he is lying to her to save face. However, Linda will never allow Willy to feel emasculated and pretends, along with him, that everything is alright.