Willy's failure to be a successful salesman means that he's forced to live out his dreams of success through his sons. Among other things, this puts an intolerable burden of pressure on them which, ironically, actually prevents them from achieving success. As a high school football star, Biff had a genuine shot at the big time. But things turned sour, and the revelation of Willy's extramarital affair with a secretary shocked Biff so much that he dropped out of summer school, seriously damaging his prospects.
As for Happy, he's never really been allowed by Willy to grow up. He's internalized his old man's fixed belief that all you need to succeed in life is to be a well-liked man. So Happy just drifts through life without any sense of direction or purpose, dissipating his energies in the pursuit of hare-brained business ideas as well as pretty young women.
In Arthur Miller's Death of a Salesman, Willy has parenting skills that are similar to his life skills. He treats both of his sons with blind optimism that they can do anything they want, ignoring their limitations and desires. He wants them to be successful and well-liked—these are the two qualities he himself strives for. However, one of his sons, Biff, does not wish for this—he would like to live out West. The other son, Happy, is so disillusioned by his father's grandiose ideas that he becomes lazy and insolent. By imposing his own dreams on his sons, Willy has made exactly the opposite of what he has imagined. Both of his sons are failures, they do not really love him, and they aren't especially well-liked.