When Slim first meets George and Lennie in Chapter 2, George makes a point of telling Slim that Lennie is not "bright," but he can lift bales of barley with ease. Then, in Chapter 3, after Slim has mentioned the pups that his dog has whelped, George has asked him if Lennie can have one; Slim agrees to do so, adding "No need to thank me about that."
George said, "It wasn't much to you, maybe, but it was a hell of a lot to him....I don't know how we're gonna get him to sleep in here. He'll want to sleep right out in the barn with 'em. We'll have trouble keepin' him from getting right in the box with them pups."
From these words of George, it becomes apparent that mentally Lennie is like a young boy eager to have a puppy and hold it close to him. So, as he and Slim talk further, Slim remarks about Lennie, "He ain't mean....I can see Lennnie ain't mean." Slim has watched Lennie with the puppy; he has also observed Lennie in the field at work. And, he has listened to George relate how he tricked Lennie and even beaten him, but Lennie never fought back:
"I've beat the hell outa him, and he coulda bust every bone in my body jus' with his han's, but he never lifted a finger against me."
Further, George tells Slim that he has noticed how the itinerant workers who travel by themselves develop a mean nature and become taciturn. But, now, he and Lennie have each other.