In the 4th section, or chapter 4, Crooks character is clearly defined. The fact that Crooks has accumulated lots of books shows that he is used to being alone because no one wants to be with him. Being into reading is one way to avoid lonliness. Then, as he begins to talk with Lennie, at first he doesn't want Lennie near him. Lennie really doesn't mean any harm, but Crooks is used to white guys harming him. This guardedness further develops Crooks' lonliness. As you get further into the chapter, you see that Crooks is almost eager to make fun of Lennie because he doesn't have anyone to do that to. He tells Lennie how easy it would be to befriend someone like Lennie because a Lennie wouldn't go tell secrets. He further tells Lennie about his family was alone. He has a few words that really demonstrate his longing for friendship which conversely demonstrates lonliness:
"The thing is, they're talkin' or they're settin' still not talkin'. It don't make no difference, no difference."
Crooks would really like someone to just "set with". But he can't because the guys think he stinks.