Throughout John Steinbeck's "Of Mice and Men" it has been shown how loneliness leads one's self to insanity
When one is accompanied by friends they're able to make just decisions such as George helping Lennie escape from weed (p. 7) or guiding him away from the pond water (p. 2-3)
"Lennie, for God sakes don't drink so much." "Lennie. You gonna be sick like you was last night."
While Lennie walks through the barn he stumbles upon Crooks who tells him about how his life used to be and how no one cared about him on the ranch. Later on reminiscing one his past as a child when he had friendship (p. 70-72)
One character where this is quite evident is George. Transitioning from a strong, confident man in the beginning of the story. To a frail husk of one nearing the end.
After George looses Lennie, Slim attempts to comfort George by saying they should go get drinks but instead of the charismatic, witty George all he says is "Yeah, a drink." (p. 107)
"Come on, George. Me an' you'll go in an' get a drink."
By examining the setting in Ch. 1 it reveals that George is clearly quite the competent character when accompanied with a friend, guiding them from danger even being a kind of guardian for Lennie. He's able to examine the situation around him and correct mistakes of others.
In Ch. 4 we get a glimpse as to another example of someone who's lost their sanity because of loneliness, Crooks. Even though his Loneliness came from how culture treated POC in this time they still end up with the same result. If someone is unable to interact they will loose their joy and sanity.
This final page wraps up on how George when abandoning Lennie, looses all of him that was left sane, as the Luger falls his mind follows. Walking slowly away from his actions and loss he joins Slim silently, as if he's a new man, stranded in his own mind because he knows he's made a fatal choice that he can never take back.