In Of Mice and Men, Crooks is an outcast, the lowest ranking male on the ranch hierarchy. He has the double-whammy of being black and crippled. Though he is very intelligent (he reads incessantly), Crooks has not only been denied the American Dream, but he has been denied any kind of community.
Crooks is a foil for Lennie. Whereas Lennie mentally disabled, Crooks is very intelligent. Whereas Lennie is physically strong, Crooks is physically disabled. Whereas Lennie has George, a partner to look after him, Crooks is lonely.
Crooks is very distrustful of whites. So says another Enotes editor:
Despite being disabled, Crooks is beaten at Christmas as part of a game. Candy relates this story with some glee, telling a newly arrived George, "If he coulda used his feet, Smitty says he woulda killed the nigger. The guys said on account of the nigger's got a crooked back, Smitty can't use his feet." The others call him "nigger" without ever considering his emotions.
Crooks himself tells how, as a child, his father warned him to not be friends with whites because they would turn on him. His father is right. Candy first holds out hope by telling Crooks about the farm. We see Crooks grab at that hope, but then when George comes back, George immediately dismisses the idea of including Crooks, and Crooks is even more withdrawn.
Whereas Lennie is the keeper of the Dream Ranch, Crooks is the realist who realizes that Lennie is "crazy" and that his dream will never be achieved.
In my opinion, Crooks is bitter because he has been mistreated all his life on account of his race. If you look at his life, you see that the men never include him in anything. He is not allowed to participate in the horseshoe tournaments or the card games. He can't go into town with the men. So of course he's bitter.
I think he is willing to talk to Lennie because he understands that that Lennie is in some way an outcast as well. He recognizes that Lennie will not treat him badly because Lennie is not like the other people and will not look down on him.