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WILLY: [the last to leave, turning to Charley]: I don’t think that was funny, Charley. This is the greatest day of his life.
CHARLEY: Willy, when are you going to grow up?
WILLY: Yeah, heh? When this game is over, you’ll be laughing out of the other side of your face. They’ll be calling him another Red Grange. Twenty-five thousand a year. (Act 2)
Willy's detachment from reality causes him to massively exaggerate the significance of Biff's high school football career. He feels the need to take any small success and blow it up into epic proportions.
WILLY: In 1928 I had a big year. I averaged a hundred and seventy dollars a week in commissions.
HOWARD: Now Willy, you never averaged—
WILLY: I averaged a hundred and seventy dollars a week in the year of 1928! (Act 2)
Willy has himself utterly convinced that he is incredible at his job, despite the obvious reality of his poor salesmanship. Howard doesn't seem to buy any of it, though. Willy's transparently deceitful nature may be part of what costs him his job in this scene.
WILLY: I’m definitely going to get one. Because lots of time I’m on the road, and I think to myself, what I must be missing on the radio!
HOWARD: Don’t you have a radio in the car?
WILLY: Well, yeah, but who ever thinks of turning it on? (Act 2)
Willy comes off looking like a fool when he attempts to lie in order to impress Howard and soothe his own insecurities. Ironically, his constant need to lie in order to make himself well liked is probably a big part of the reason that people don't really like him.