John Steinbeck wrote Of Mice and Men with the intention of turning it into a stage play, which he did in 1937, the same year the book was published. This fact explains a great deal about the construction of the novella. He wanted to make the adaptation as easy and as faithful to the original as possible. This meant relying heavily on dialogue, since most stage plays are almost totally dependent on the spoken word. Much of the exposition is conveyed through conversations between the characters, and this is especially true of the conversations between George and Lennie.
Steinbeck probably hit on the idea of making Lennie feebleminded so that George would have to keep explaining everything to him--and at the same time explaining everything to the reader and to the future audience at the stage play. In the opening chapter George tells Lennie where they are going, where they have been, describes the trouble they had in Weed, and instructs him to come back to this campsite by the river and hide if he should get into trouble at their new place of employment. Steinbeck also uses Lennie's imbecility as an excuse to have George explain the shared dream of owning their own farm, which is the main motif of this story.
Lennie pleaded, "Come on, George. Tell me. Please George. Like you done before."
George's voice became deeper. He repeated his words rhythmically as though he had said them many times before. "Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place. They come to a ranch an' work up a stake and then they go inta town and blow their stake, and the first thing you know they're poundin' their tail on some other ranch. They ain't got nothing to look ahead to. . . . With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us. We don't have to sit in no bar room blowin' in our jack jus' because we got no place else to go. If them other guys gets in jail they can rot for all anybody gives a damn. But not us."
As it is, George and Lennie make a grotesque and ill-fated partnership. Nobody can understand why George burdens himself with an overgrown child who kills little animals by petting them to death and who nearly got them lynched in Weed for trying to pet a girl. Even George can't understand why he does it--and if George can't understand it, how can the reader be expected to understand it?
"You can't keep a job and you lose me ever' job I get. Jus' keep me shovin' all over the country all the time. An' that ain't the worst. You get in trouble. You do bad things and I got to get you out."
George's only substantive reason for putting up with such a burden, as we learn through still more dialogue, is that he made a promise to some "Aunt Clara" to look after Lennie. It is an insufficient motivation, but Steinbeck was wise not to try to pile on more excuses because they would only call attention to the bizarre partnership and the improbability of their being able to realize their "American Dream."
It is easy enough to visualize George and Lennie but hard to understand their relationship, and it is hard to sympathize with Lennie because he seems almost like a monster and because he is always killing little animals--either accidentally or perhaps accidentally on purpose..