Crooks lives apart from everyone else because he is black.
Crooks is the “lean stable buck.” He sleeps in the harness room because he is black, and in those days during the Great Depression, segregation was the norm.
Crooks, the Negro stable buck, had his bunk in the harness room; a little shed that leaned off the wall of the barn. On one side of the little room there was a square four-paned window, and on the other, a narrow plank door leading into the barn. (Ch. 4)
Crooks lives and sleeps alone, and in the description of his home it says he “could leave his things about” because he lives alone. However, he is often lonely. He feels isolated, like many of the different people on the ranch. In his case, it is because of his race. He is a cripple, and he is a permanent resident of the ranch, unlike most of the other men. He has “accumulated more possessions than he could carry on his back” (Ch. 4).
Steinbeck spends a lot of time describing Crooks’s living quarters. For instance, he reads. He has spectacles, magazines, and “a few dirty books on a special shelf over his bunk” (Ch. 4). This means that he has a lot of time to himself. He also notes that Crooks keeps his room neat (although things are strewn about) because it is swept.
Even though Crooks is lonely, he feels intruded upon when Lennie enters.
Crooks said sharply, "You got no right to come in my room. This here's my room. Nobody got any right in here but me. … You go on get outa my room. I ain't wanted in the bunkhouse, and you ain't wanted in my room." (Ch. 4)
His defense of his space likely comes both from living alone for so long, and for having to put up with racism. He doesn’t understand yet that Lennie is not like other men. Lennie really has only come to see the puppy. Crooks has already tried to get rid of Lennie once before when he was looking at the puppies, telling Slim he was bothering them. He wants to keep his space his space.
The fact that no one understands that he is defending it is an example of the racism that Crooks faces. He has no other place to go but the stable, but no way to keep anyone out of it. It is his room, but it is a public place too.