The rabbits are emblematic of a simple and idyllic life, but rabbits are a fraught symbol: we know Lennie is excited about them because they’ll be furry and lovely to pet, but we also know that Lennie tends to hurt whatever he pets. This doesn’t bode well for him and he knows it, hence the large, scary, vitriolic rabbit at the end of the story. That rabbit announces that Lennie isn’t fit to lick the boots of a rabbit, but that the bunny comes from Lennie’s own mind suggests that he knows deep down he’ll never have his dream. The fact that rabbits never actually appear in the book (though they figure so heavily) highlights the unfortunate reality that Lennie’s dreams can never materialize.
Mice represent the false hope of a safe space for Lennie. The title is a good hint that mice are important here, but the first mouse we encounter is a dead one. Actually, it’s a dead one that Lennie keeps in his pocket to pet. This is a huge clue: Lennie doesn’t care much about death, and he’s more concerned with comfort – remembering this makes Lennie’s death a bit more palatable. He’ll be more comfortable if dead by his friend’s gentle hand than with a violent end from Curley or the cage of an asylum.
Mice are a source of comfort for Lennie, as he links them to his nice Aunt Clara. In fact they’re all he really remembers of her. But in...