Willy Loman is a typical high-strung salesman, always looking for the bottom line, and always trying to get ahead despite his lack of ability. While he is generally competent, he tends to dream higher than his actual abilities, and so he strives for an excellence he does not possess instead of being satisfied with mediocre successes. His motivations are to become independently wealthy like his brother, and to be admired by the other salesmen and public figures he sees in his daily life. The play shows that he could have been happy with his life, if he had not been driven to different ideals; his family comments that he seemed to truly enjoy building, rather than selling:
CHARLEY: Yeah. He was a happy man with a batch of cement.
LINDA: He was so wonderful with his hands.
BIFF: He had the wrong dreams. All, all, wrong.
(Miller, Death of a Salesman, kevindeweese.com)
In this sense, his motivations changed him from a realistic man to an idealistic, almost mystical man. He didn't believe in his ability to be successful based on his own merits, but he did believe that he could somehow stumble on luck if he just worked hard enough. Had he been more grounded in reality, he could have become a contractor or even just a laborer, but he thought that the difficult life of selling would be enough to achieve his ideal of greatness.