Of Mice and Men is set during the Great Depression, a time of unmitigated poverty in the States. George and Lennie are two migrant workers. Steinbeck uses animal comparisons to describe them, as if to suggest that the desperate poverty in which they live has reduced them to this state. Lennie is described as a clumsy, stupid bear and George as a scheming, clever rodent of an individual. Animal similes are used in particular with Lennie: he drags his feet "the way a bear drags his paws" and drinks from a pool "like a horse".
Apart from their physical description, they are also opposite in outlook. George is incredibly suspicious and cynical of the world, whereas Lennie sees the world as a beautiful playground, full of fluffy toys for him to play with. George sees darkness whilst Lennie sees light.
Although both George and Lennie talk about life without each other, these are seen to be hollow threats. Despite George's complaints, it is as if he needs Lennie's innocence just as much as Lennie needs George's worldly-wise attitude to survive. Together they complete each other and as a pair they are more than if they were just individuals.