Why does Crooks think that George and Lennie's dream is just foolish?
Crooks believes that George and Lennie's plan to have a small farm is just a pipe dream because he has seen "hundreds of men" come through with the same hopes and never have the dreams been fulfilled, nor have any others, for that matter.
When Lennie talks of his and George's plan to own a piece of property, Crooks scoffs at this dream, telling Lennie he is "nuts." Scornfully he says,
I seen hundreds of men come by on the road an' on the ranches with their bindles on their backs an' that same damn thing in their heads....They come, an' they quit an' go on; an' every damn one of 'em's got a little piece of land in his head. An' never a...one of' em ever gets it. Just like heaven....Nobody never gets to heaven, and nobody gets no land. It's just in their heads.
Unlike Lennie, Crooks is well aware of the economic state of the country during the Great Depression; consequently, he understands that for a transient worker to save money is nearly impossible. In addition, Crooks probably views life through a negative lens since he was once treated fairly by whites when he was a boy on his father's chicken farm in California, and there were no other "colored families" for miles. But, now he is marginalized and when he says something he is given no credibility because, as he tells Lennie, "it's just a nigger sayin' it." Now, in his resentment of having had the dream of equal opportunity that was taken from him, it is not unusual for Crooks's thinking to be negative. After all, his idea of equal treatment from all was "just in [his] head"--much like other men's dreams of owning land.
Crooks thinks this because, he says, he's seen too many people with similar dreams which just fail. He thinks that dreams never come true for anybody. His life experiences have made him rather bitter and cynical; he's come to the conclusion that life is a hard and dreary business. After all, he has been a victim of racism all his life, and he has no company at the ranch, being the only black worker there. Furthermore, like Candy, he is disabled after an accident. It is not surprising if his outlook on life tends to be negative. Even so, he is briefly drawn into considering the project along with Lennie and Candy, being carried away by their enthusiasm. However, he soon goes back on this, and returns to his lonely, bleak life in his little room.