The Answer would be "The variety in nature"
Fable
Think of the hare and tortoise , it’s a fable about not being best at everything and to take your time. It was never directly said but you can figure it out by reading it from beginning to end.
Crooks lives by himself because hes black. the author writes and shows us to show effect of discrimination between him and the rest of the guys its ackward because no ones ever been in his room before,since lennie went in,candy came in looking for him noticing how long hes been on the ranch yet never stepped foot in crooks room.
The overall theme is a struggle with one's emotions, and not being able to control your desires or even know how you feel.