"A guy goes crazy if he ain't got nobody"
Crooks
Isolation
This quote suggests that being isolated from others and socialising, it can make one out of one's mind because they aren't learning new things and aren't communicating to another person about how they may feel. In this case, Crooks is out of his mind because he is discriminated against, diffused from the others as well as segregated from others because of his race. He is the only character in the play who really deserves to be able to talk to somebody about how he feels. When this opportunity had risen, Crooks was overwhelmed and told Lennie that he "Don't have no right to come in here". As innocent as Lennie is, he let this go over his head and proceeded.
An alternative interpretation of the quote is the fact that it is a double negative. This means that what the quote is really saying is that 'A guy goes crazy if he's got everybody'. This means that by being friends with everybody and being overly popular, it can 'get to your head' and make one feel prestigious and above everybody else, for example, Slim. The reader feels a sense of tension or slight jealousy when Slim says "funny how you an' him string along together", this is because friendship was such a rare thing to happen to men in ranches, even George says "Men like us that work on the ranches, are the loneliest in the world". Slim may be envious of this friendship because, although he is popular amongst everybody in the ranch, he doesn't seem to have somebody he likes to spend the majority of his time with like George does with Lennie. In this sense, he blends into the background just like everybody else - this can be further proved by the fact that he is wearing the same clothes as everybody else on the ranch and he is not wearing anything to define his popularity; Slim, and others around him, were described as "wearing a short blue denim jacket". Slim is said to "look through George and beyond him". This tells us that Slim can understand George's background and everything he has to do for Lennie which gives him a better understanding of Lennie as a person. Slim may have realised that Lennie is stronger than he seems but kept this to himself because he is respected meaning that he has to respect others around him and not 'spread rumours', etc. Steinbeck also introduces Slim and Curley's wife's 'friendship' as quite flirtatious - could this be because Slim is more powerful than Curley? (Slim is described as "tall" and Curley is described as "small", opposites). To add to this, Slim was said to have been "giving her the eye" which means that there could be a secret relationship between the two or that they could be hiding something.
I feel that what the two could be hiding is the fact that Slim had set Curley's wife up into going into the bunkhouse to visit Lennie and the men because Slim knew that Lennie was weak-minded enough to follow her around whilst he is alone. Curley's wife takes Lennie into the barn which could have been what envious Slim wanted all along - for Lennie to kill Curley's wife, so that he could have George to himself. When Lennie is killed, I get a sense that Slim was jealous of George's relationship with Lennie and how they looked after each other, emphasis on how Slim, George and the guys on the ranch went out for a few nights without Lennie. Slim was slowly but surely taking George away from Lennie with a cunning plan that left Lennie for dead. All of the men were curious about George and Lennie's relationship, but not as much as Slim; he may have been considered the most popular and the "prince of the ranch", but in other ways he was lonely and all he wanted was a real companion.
This is also somewhat supported when Lennie finds and is gifted animals as a treat to make him feel like he's got a friend until he kills it. This could mean that Lennie is looking for a particular companionship (identical to George's), meaning that anything that gets in his way will be destroyed, for example, the mouse and the pup that Slim gives Lennie. Slim may have given Lennie this pup to give him the comfort Lennie has been asking for throughout the novella up until this point, little did Slim realise that having the pup was only a temporary comfort and it would not make Lennie leave George's side. Lennie is loyal, almost like a pet - Steinbeck continuously uses an animal theme to describe Lennie throughout the novella and Lennie being loyal may be why.
^I've just written this and I feel that I may be thinking a little too outside of the box.. but it's definitely a view that you could take into consideration.