Why does Slim think that two men traveling together is funny?

Slim thinks that it is funny because "hardly none of the guys travel together"(39). Slim is not used to people traveling with other people. I think that Slim was surprised because George was traveling with Lennie who is not the brightest. Slim heard the stories about George and Lennie traveling together and I think that he thinks that Lennie would be tough to travel with and just be in the way of George. Another reason why Slim thought it was funny is because he thought that Lennie was crazy but George says "He's dumb as h***, but he ain't crazy"(39). George is saying that Lennie is stupid but he is not crazy which I think is the opposite of what Slims intentions were.