In Chapter 3, Slim says to George, "Funny how you an' (Lenny) string along together". By using the word "funny", Slim means "unusual" rather than "humorous", and his comment is a "calm invitation to confidence", an encouragement to George to share his feelings on his relationship with Lenny.
George responds defensively at first to Slim's observation, but Slim explains himself, saying, "Oh, I dunno. Hardly none of the guys ever travel together. I hardly never seen two guys travel together. You know how the hands are, they just come in and get their bunk and work a month and then they quit and go out alone. Never seem to give a damn about nobody. It jus' seems kinda funny a cuckoo like him and a smart little guy like you travelin' together".
Slim's comment and his quiet and receptive demeanor actually do encourage George to explore and talk about his relationship with Lennie. George reveals how he promised Lennie's Aunt Clara that he would take care of him, and how Lennie got in a scrape over in Weed. George says that Lennie is a lot of trouble, but appreciates the companionship they provide for each other. He realizes they are both lucky not to be alone. Slim listens to George's musings, and understands the situatioin, repeating several times that although Lennie isn't bright, despite whatever he has done, "He ain't mean" (Chapter 3).