Candy explains that Crooks is a loner who'd rather stay to himself, but later on the reader understands that he really has no other choice. Candy has been ostracized from the other farm hands because he is a Negro and because he is disabled. (He had injured his hand some time earlier on Curley's farm while running a cotton gin.)As he can no longer do the same kind of labour as the other men, Candy has been delegated kitchen dutes and other domestic tasks. This is humiliating for him, and he has moved into the harness room instead of sharing lodgings with the other workers.
Candy retreats into a world of his own and substitutes normal companionship by reading. He later confesses to Lennie how terribly lonely he really is.
Of Mice and Men deals with the subject of friendship (portrayed in the relationship between George and Lennie), but equally important are themes around alienation and loneliness.