Crooks wants to have someone who care about him, he also want to be friend with the guy on the ranch. He also wants to play card with them after work everyday with them and when he heard Candy talked to Lennie about them wanting to get their own ranch, he even want to join and help them.
Crooks feels like he doesn't belong on the ranch and hang out with the guys or play cards with them in the bunk house because he's a colored man so he just stay in the barn and read books and have no one to talk to.
Crooks decided that his dream isn't worth pursing when he talked back to Curley's wife and she said she's going to lynch him with he keep talking back to her. That's when he realized that he's just a black man and his voice means nothing on the ranch and in society and he decided not to join George, Lennie and Candy's on their ranch any more because he's just a black man, and they probably not going to want him on their ranch.