The best place to look for quotes about loneliness occurs in Chapters 2 and 3 of the novel when George and Lennie end up hanging out with Crooks, the black stable buck, and then with Curley's wife.
The most famous quote about loneliness from Of Mice and Men is in Chapter 2:
"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 ... I tell ya a guy gets too lonely an' he gets sick."
Chapter 3 is another good place to find quotes about loneliness. This chapter focuses on Curley's wife. She says the following:
"Why can't I talk to you? I never get to talk to nobody. I get awful lonely."
"I get lonely ... You can talk to people, but I can't talk to nobody but Curley. Else he gets mad. How'd you like not to talk to anybody."
While all the men on the ranch experience a level of loneliness that's not capable of being comprehended in modern society, the level of loneliness experienced by Crooks and Curley's wife is unique. Crooks is the only black man on the ranch. He's not welcome by the white ranch hands because of his skin color. He is truly alone.
Meanwhile, the only other character in this novel who is truly alone is Curley's wife. Sure she's married to the boss's son, but she's clearly not receiving the attention she needs. She is the only woman in the entire novel, which makes her loneliness, like Crooks's loneliness, unique.