Friendship 1: Despite George's impatience and annoyance with Lennie, and his remarks about how easy his life would be without him, he still believes that:
"Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place....With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us." Chapter 1, pg. 13-14.
And Lennie finishes:
"An' why? Because...because I got you to look after me, and you got me to look after you, and that's why." Chapter 1, pg. 14.
The kind of life these men lead, moving all over the country, never knowing anyone very long, and having very little to call their own, is intensely lonely. Even if Lennie is not very bright, he still listens to George, and he remains the one constant in George's transient life. For this George is grateful.
Friendship 2: Slim comes across very differently than the other men. Friendly and understanding, he invites George into conversation. When discussing how George and Lennie travel together, Slim remarks:
"'Ain't many guys travel around together,' he mused. 'I don't know why. Maybe ever'body in the whole damn world is scared of each other.'"Chapter 2, pg. 35.
Slim is much more open than most of the men on the ranch, and a marked contrast to Curley, whose can only communicate with fighting. Curley will push his wife away, choosing to go visit prostitutes rather than work on their marriage, whereas Slim attempts to construct a relationship with George the first chance he gets. The men have a deep respect for Slim, and his opinion is the final word on any subject.
Friendship 3: When George tells Slim how he used to play tricks on Lennie, beat him up, and generally abuse him for his own amusement, we get a very different picture of Lennie and George's friendship. George admits one reason why he behaved such:
"Made me seem God damn smart alongside of him." Chapter 3, pg. 40.