Curley picks on Lennie the most because he is jealous of his size. Curley thought Lennie was laughing at him but Lennie was laughing at something completely different. This is what triggered the fight.
George tells Lennie to stay away from Curley because he is nothing but trouble. Curley is somewhat a threat to George and Lennie. He is mean and he's the bosses son so he thinks he can run everybody.
Crooks is an African American worker at the ranch that tends the horses. He represents loneliness because he is treated different from the others and lives in a room by himself. He isn't welcome in the bunkhouse because of his skin color. "S'pose you didn't have nobody. S'pose you couldn't go in the bunkhouse and play rummy 'cause you was black." "I tell ya a guy gets too lonely an' he gets sick." These quote from Crooks reveal that he doesn't like being by himself in a room and he wishes he could have someone with him. Crooks could live with George and Lennie at the farm so that Crooks doesn't have to be lonely in a room by himself.
Candy is a lonely guy in the story. After he lost his hand he could no longer work like the other men so he had to become a swamper. The only thing Candy had was his dog. "You wouldn't think it to look at him now, but he was the best damn sheep dog I ever seen." After Carlson shot his dog Candy had no motivation to go on. This forces him to talk to Lennie and George so that he won't be lonely. If George and Lennie let him live at the farm with them this could help him overcome his loneliness.