In the novel Of Mice and Men, Slim has something Lennie wants very badly.
Lennie has great affection for things that are soft. He thoroughly enjoys petting and touching soft things. As seen in the opening of the text, Lennie also has the propensity of killing the small animals he picks up to touch.
The dream that George and Lennie have about owning their own land is ever the more appealing based upon the fact that George has promised Lennie that he can keep rabbits.
That being said, Lennie really wants one of the puppies Slim's dog just had. When Lennie hears about the puppies his knowledge of their fur and softness fills his mind. He feels as if having a puppy will make him ultimately happy.