Crooks is isolated, powerless, and embittered by his social position. These characteristics also describe many of the other characters in Of Mice and Men.
First we might look at the systematic or demographic isolation and discrimination that defines Crooks' position on the ranch. He is the only African-American on the ranch and is not allowed to live in the bunk house. He expresses his bitterness at this discrimination to Lennie and says that this has been the case for him his whole life.
Crooks, the despairing old Negro stable worker, lives alone in the harness room, ostracized from the ranch hands.
Isolation according to social position oppresses George and Lennie as well, as poor migrant workers, oppresses Candy as an elderly man of little physical power, and Curley's wife as the sole, friendless woman on the ranch.
Taking this into account, we can say that Crooks represents one way that society is a separating force, isolating individuals and pressing them into codified, well-defined limitations of identity.
Friendship for Crooks and Curley's wife is impossible. This is a fact in the book directly resulting from the social position of each character, which is to say that it is a result of discrimination characteristic of American society at the time.