These "mirror images" are mainly examples of foreshadowing. Some of Steinbeck's use of the images is simply to build suspense, but he also relies upon them to demonstrate the universality of man's plight and various forms of isolation.
-Lennie's uncontrollable strength causes him to snap his puppy's neck and eventually Curley's Wife's neck.
-When many of the men go into town to visit a cat house, Candy, Lennie, and Crooks stay behind. Each of them represents an isolated outcast: Candy because of his age and alleged uselessness, Lennie because of his mental handicap, and Crooks because of his race.
-Lennie and George are run out of town because of Lennie's problem with a girl; likewise, Lennie causes another problem with a girl (Curley's Wife) on the Tyler Ranch.
-Steinbeck's description of the river and its inhabitants in the first chapter and last are almost identical. The animals scurry away from man each time. George and Lennie discuss their dream in both scenes, but in Chapter 1, they are filled with hope about their dream; in Chapter 6, George knows that the dream is elusive.