SparkNotes comments that Crooks, in chapter four of "Of Mice and Men," speaks about his family and growing up on a chicken farm. He states that he played with white children at that time, and that his family was the only black family around for miles.
Crooks' father tried to instill in Crooks the importance of watching out for white people. While they might seem nice at times, they are not always that way towards black people, he warns.
The importance of this lesson that Crooks did not understand as a child is to compare his father's advice to Crooks' current situation on the ranch in Soledad. Crooks talks to Lennie after Lennie refuses to leave Crooks' bunk. Although he is not sure that Lennie is really listening, Crooks confides in Lennie about how unfair it is that he must stay in the bunkhouse most of the time by himself. He is forced to read books because there is nothing else for a black man to do on the ranch. He says how lonely he is and how sick a man gets if he is lonely.
Crooks then ties his feelings into what seems like jealousy in regards to Lennie and George's relationship. They travel together and are never lonely. Eventually, Lennie explains about the dream ranch he and George are going to run, and Crooks says he really wants to work on that ranch too, to finally have companions and be treated like an equal. But, like his dad always told him, he needs to watch out for white people, as George implies that this plan is not going to work.