Willy has had a competitive relationship with Charley ever since they have been neighbors. This has been for a long time, long enough for Biff and Happy to think of their father's friend as Uncle Charley. The card games between Willy and Charley symbolize their competitiveness. Charley has obviously been much more successful over the years. He can give Willy fifty dollars every week without expecting to get repaid. But Willy insists that the money is only a loan which he will positively pay back. He does not want to admit to Charley or to himself that Charley has beaten him in the game of life. If he were to go to work for Charley, this would be the same as an admission of defeat.
Then when Willy has to realize that Charley's son Bernard has been vastly more successful than his own son Biff, it would be utterly impossible for Willy to accept Charley as his boss, someone who would assign him to a territory and keep giving him orders. Willy's refusal to concede defeat symbolizes the way capitalism makes everyone compete, whereas it would be more sensible for everyone to cooperate.
Essentially, it is Willy's inflated and unrealistic pride that keeps him from accepting Charley's generous and very sensible job offer. When Willy is having his big argument with Biff towards the end of the play, he says, "I'm not a dime a dozen. I'm Willy Loman." That is both comical and pathetic. He certainly is Willy Loman—a nobody, a loser, a "hard-working drummer who ended up in the ash can," as Biff calls him.
Willy seems to have no choice but suicide. He can't handle the New England territory. His boss won't assign him to a territory close to home. He can't bring himself to go to work for Charley. He can't keep borrowing fifty dollars a week from Charley indefinitely, because the total debt would mount so high that he could no longer pretend to himself that he would be able to pay it back. Neither of his sons can earn enough to support him and his wife, and he is probably too proud to accept money from them anyway.