In Death of a Salesman, Happy says several times, "I'm losing weight, you notice, Pop?" Which American myth is he bringing up here?
Happy is perpetuating the myth that a successful business person must not be overweight and must look physically fit. At one point Happy brags to Biff that he "can outbox, outrun, and outlift anybody in that store." However, Happy says many things that he later contradicts, and viewers understand that much of his bragging is pure fantasy. Happy's assurances to his father that he is losing weight are meant to help Willy have faith that Happy will be successful in business.
Willy himself is still trying to figure out the formula for being a successful salesman after all the years he has spent in his career. As he speaks to Linda, he confides that "people don't seem to take to me." He believes people he meets while on his sales calls are laughing at him. He first suggests it is because he talks too much or tells too many jokes. Then he states, "I'm fat." He relates an incident where he heard another salesman call him a "walrus." Willy says he "cracked him right across the face." Willy was upset by the remark because it made him feel unsuccessful and unworthy.
Since neither Happy nor Willy has been able to understand that success in business--as in life--is based on internal character rather than on external appearances, each of them perpetuates the myth that a successful person must not be overweight but must be physically fit.
Happy wants to stay young, virile, and fit forever. Our culture values the myth of perpetual youth. Happy knows his glory days are behind him. It is especially hard for poor Happy to let his youth go because his father doesn't want to let them go either.
Happy looks to his father for confirmation. To me, the repeating of the question "I'm losing weight, you notice, Pop?: is his way to keep the myth alive. To me, it is not much differnt than Santa Claus. "Does Santa really exist?" "Of course, son."
Because of Willy and Happy's complicit arrangement to stay within myth, it's no wonder that it is Happy at the end who cannot deal with Willy's death. Who will tell him he's still young now?