How does Steinbeck explore the theme of loneliness through the character of Crooks in Of Mice and Men?

Crooks is the African-American stable hand on the ranch.  He is lonely because his race and his position place him on the bottom of the food chain.  He does not bunk with the other guys, instead living on his own in the stable.  He also doesn’t socialize with them.
The first introduction of Crooks demonstrates his loneliness.

The door opened quietly and the stable buck put in his head; a lean negro head, lined with pain, the eyes patient. "Mr. Slim."
Slim took his eyes from old Candy. "Huh? Oh! Hello, Crooks. What's'a matter?" (Ch. 3) 

The only person who is nice to Crooks is Slim, because Slim is confident in his position and able to treat anyone with respect.  Crooks is always on the outside looking in.  As an African-American, he is a complete outcast.  He is also one of the few permanent residents at the ranch, living in the stable. 
The description of Crooks’s room and his possessions are symbolic of his isolation.  His possessions can be scattered about because he is the only one who is ever in there.  There is no need to clean up or consider anyone else’s needs.  He has also been there, alone, for a long time. 

And scattered about the floor were a number of personal possessions; for, being alone, Crooks could leave his things about, and being a stable buck and a cripple, he was more permanent than the other men, and he had accumulated more possessions than he could carry on his back. (Ch. 4)

Crooks is described as a "proud, aloof man," but this is also self-protection.  He knows that people avoid him, so he avoids them.  He tells Lennie to stay out, because he wants his own space.  Lennie doesn't understand racism, because he has the mind of a child.  He just wants to see the puppies.