Lennie and Crooks have an interesting relationship because they do not adhere to societal norms. Because Crooks is the only black man on the ranch, he has his own living quarters. He has a good working relationship with the men, but it does not go much further beyond that. He is not a part of their card games and does not go to the "saloon" with them.
In Chapter 4, when all of the men have left, only Crooks and Lennie have been left behind because they do not quite fit in with the group of men. Lennie wanders into Crooks' living quarters, simply because he is lonely and doesn't have anyone else to talk to. He has no idea that white people do not just "hang out" in Crooks' room for no reason. Crooks' initial reaction is to distrust Lennie. He tries to explain to Lennie that he has no place being there, but Lennie doesn't understand, so he continues to stay and ramble to Crooks. Eventually, Crooks softens and invites Lennie to come in and sit.
Crooks and Lennie are brought together by their loneliness, and this is perhaps their only common bond. Their conversation in Crooks' bunkhouse takes a bit of a sour note when Crooks asks Lennie to consider whether George might just take off and leave him there, which upsets Lennie. Most likely, Crooks was not trying to upset Lennie. He brings this issue up because he is projecting his own loneliness and bitter views of the world to Lennie. He believs that each man is out for himself, and for a moment wonders why George would not be the same.
For a moment, Crooks is allowed to have some hope for his life during his conversation with Lennie in his room. Until this point, he has accepted his place in society as a black man who can do nothing but keep to himself and do his job. When he talks to Lennie, Candy walks into the room and they begin discussing their dreams for the future. Crooks, getting caught up in the fantasy and forgetting his place, asks if he can join their plan. He is able to believe it can happen only briefly, because soon Curley's wife walks in and tells him his place.
Loneliness and the desire for a place to call home are the two things that bring Crooks and Lennie together, if only for a moment. Lennie's blindness to racial inequalities allows him to open up to Crooks, and Crooks' desire for someone to talk to allows him to open up to Lennie. Though this bond is short-lived, it is a significant event in the novel because it highlights the loneliness of ranch life and shows that though all men had dreams, these dreams were not always realistic.