In The Death of a Salesman, what compels Willy Loman to commit suicide, for the sake of his family?

Arthur Miller’s Death of a Salesman is about the gradual but steady degradation of a human being. Willy Loman has spent his life on the road working as a traveling salesman. His perpetually suffering wife Linda takes care of their house while their two grown sons, Biff and Happy, aimlessly stumble through life. Willy treats Linda poorly and is frequently critical of Biff for the latter’s failure to find his place in life. Willy, however, is increasingly forced to come to grips with the approaching end of his career and life. He and Linda have struggled to stay above water, never quite able to reach their goal of financial security. In one exchange between the two, Willy and Linda discuss the approaching attainment of one major goal, the final mortgage payments on their home:

WILLY: Whoever heard of a Hastings refrigerator? Once in my life I would like to own something outright before it’s broken! I’m always in a race with the junkyard! I just finished paying for the car and it’s on its last legs. The refrigerator consumes belts like a goddam maniac. They time those things. They time them so when you finally paid for them, they’re used up.

LINDA (buttoning up his jacket as he unbuttons it): All told, about two hundred dollars would carry us, dear. But that includes the last payment on the mortgage. After this payment, Willy, the house belongs to us.

Willy kills himself. He has fallen asleep at the wheel in the past, but this time he deliberately crashes his vehicle so that Linda can use the life insurance payment to finally pay off the mortgage. It is the final act of desperation by a man beaten down by life. Willy is being marginalized by his job and the secret buried deep within himself. His secret, namely, his own extramarital affair, was accidentally discovered by his son and has torn at the moral fabric of the father-son relationship. Willy has reached a point where he believes he has nothing to gain by continuing to live the only life he has known. The business contacts he has maintained over the years have proven ephemeral and, in the end, only Charley is there to mourn Willy’s passing.  Willy’s funeral is attended only by Willy’s family and Charley despite Willy’s protestations over the years of being rich in personal relationships. With the family in disarray, constant financial pressure, and the specter of death as a pauper staring at him in the mirror, Willy reacts the only way he can contemplate: suicide. While Willy’s death enables Linda to pay off the mortgage, his death is more than financial in nature. In the play’s final scene, Linda expresses her surprise at Willy’s act just when they were on the cusp of paying off the mortgage. Charley’s response captures the essence of Willy Loman and why he killed himself:

LINDA: I can’t understand it. At this time especially. First time in thirty-five years we were just about free and clear. He only needed a little salary. He was even finished with the dentist.

CHARLEY: No man only needs a little salary.

Willy was a lonely, bitter man whose failure as a salesman and as a father haunted him, so he escapes through death.