In chapter 4 of Of Mice and Men we learn about the life of Crooks, who is the only black field hand at the farm. He is described as a man who seems to be well-organized since he keeps his room in good order. He is the only field hand who has his very own room. He is also a man who is aloof and seems to enjoy being alone.
This comes as a consequence of having been always pushed away by the rest of the field hands due to the color of his skin. The farm hands just do not seem to connect with him, nor he with them. As a result, Crooks is a man who has developed a defense mode which makes him rough, grouchy, and seemingly happy to be a loner.
Yet, when Lennie enters his apartment after being, himself, left alone at the farm, Crooks somehow becomes somewhat humanized by his presence. This is perhaps because Crooks may not be as happy to be always alone as he claims to be. After all, he does invite Lennie to sit down and talk for a while.
However, when Crooks hears Lennie talk about the dream of owning a farm with George, his pent-up frustrations, anger, and rancor that has been built up ever since his earlier years, seem to flair up. This is when Crooks begins to play a mind game with Lennie, manipulating his thoughts into thinking that George may never return, after all: That the dream may just be a lie.
The significance of this is twofold: First, it is a way for Crooks to use catharsis to vent out his own broken dreams and his overall disillusion with life. Why should he be the miserable man while a simpleton like Lennie-a simpleton representative of white prejudice at that- is sitting there talking about dreams of living off the "fat of the land"? Second, annoying Lennie is part of Crooks's own passive-aggressive personality: Pushing buttons is what makes him enjoy seeing others get mad and frustrated. After all, misery loves company.
Therefore, Crooks uses this technique to bring Lennie down to "reality" not because he cares for Lennie, but because, in principle, he feels that it is not fair that, while he has left all his dreams die, other people have the opportunity to live them. This is typical of someone who is angry and frustrated. It is not a justifiable behavior but, in Crooks's case, it is almost understandable.