why does charley visit? how does he feel about willy? and why do they insult eachother?

1) He comes over to play cards.

2) He cares about Willy and wants to help him; he offers him a job.

3) Willy starts the insult session. He's humiliated and insulted that Charley would offer him a job. They insult each other over nonsense.

"Willy asks Charley why Biff is going back to Texas, but Charley tells him to let Biff go. Willy talks about the ceiling he put up in the living room, but refuses to give any details. When Charley wonders how he could put up a ceiling, Willy shouts at him that a man who can't handle tools is not a man, and calls Charley disgusting. "

http://www.gradesaver.com/death-of-a-salesman/study-guide/section6/