Crooks talks to Lennie as they both can relate to one another due to the fact that they both are serparted from the other ranch workers. Crooks is separtated due to the fact that he is black and Lennie is too because he has a learning disability. This is also why Curley's wife talks to him as well because she is female and is also seen as lower due to the socail hierarchy.
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Crooks talks to Lennie in "Of Mice and Men" because he is lonely and looking for someone to talk to. He sees Lennie as a fellow outcast and takes the opportunity to connect with him, expressing his own feelings and experiences as a black man living in a racist society.