Why does Crooks want to be a part of the plan to buy a ranch?
Crooks would like to be part of the plan to buy a farm because he has been terribly lonely and if he lived with George and Lennie and Candy, he would have others to be around; also having a plan for his old age would give him security.
It is apparent that Crooks resents being marginalized on the ranch. When Lennie first approaches him, Crooks rebuffs his friendly gesture out of a defensiveness, and he tells Lennie, "You got no right to come in my room....Nobody got any right in here but me." But, the child-like Lennie continues to stand nearby and smiles. Finally, Crooks realizes that Lennie means no harm, so he lets him come in.
At first, Crooks taunts Lennie, telling him George may not return; however, when Lennie becomes too upset, Crooks reassures him that George will, indeed, return. Then he adds reflectively,
"Maybe you can see now....A guy needs somebody--to be near him....A guy goes nuts if he ain't got nobody. Don't make no difference who the guy is, long's he's with you. I tell ya," he cried," I tell ya a guy gets too lonely an' he gets sick."
Clearly, Crooks perks up when he can talk to another man, and he is excited about the possibility of being able to join in on the dream of George and Lennie's owning a little farm and having companionship and some security in his old age.