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First, we should say that all the men on the ranch respect Slim. So, it is not odd that George respects Slim as well.  In one passage, Slim is even described as Godlike.

George looked over at Slim and saw the calm, Godlike eyes fastened on him.

As Slim and George interact with one another, George realizes that Slim is a decent guy, probably the most decent man on the ranch. When Slim talked with George, Slim was friendly and kind.  George, therefore, responded well to Slim. Here is one example of an early conversation between Slim and George.

 “You guys travel around together?” His tone was friendly. It invited confidence without demanding it.

George also appreciated Slim's ability to see that Lennie was a nice person. In George's eyes, Slim had depth.  He did not just see Lennie based on the exteriors.  He could see that Lennie had a good heart.

“He’s a nice fella,” said Slim. “Guy don’t need no sense to be a nice fella. Seems to me sometimes it jus’ works the other way around. Take a real smart guy and he ain’t hardly ever a nice fella.”

Finally, Slim was the only one on the ranch that knew and understood why and what George did at the end.  In short, Slim understood George's predicament.

Slim said, “You hadda, George. I swear you hadda. Come on with me.”

In light of these points, George respected Slim.