Wrightkd just gave you a good one, and it's pretty straightforward for one to interpret.
Another good one occurs when in chapter 4 the guys go into town to Ol' Susie's place. Lennie wanders into Crooks place and Crooks challenges the relationship of Lennie and George. Lennie says:
I been with George a long time. He'll come back tonight-"
This shows the confidence Lennie has in George. Although, moments later he questions if George really will come back. Lennie relies on George as a father figure who takes care of him. He would be crushed if George didn't return. Crooks reaffirms for him:
You got George. You know he's goin' to come back. S'pose you didn't have nobody.
Crooks is trying to make Lennie realize how much he should value having the relationship with George that he does have.
George mentions that guys who work on ranches are the 'loneliest guys in the world'. Although he goes on to argue that he and Lennie have each other and so are not lonely, this quotation is interesting because it introduces the theme of loneliness. Most of the people they meet at the ranch the following day are lonely, like Crooks the Negro stable-buck who is isolated because of his colour, Curley's wife, who is in a loveless marriage with no friends around her,and Candy, who loses his only companion when his dog is shot. We realise how important George and Lennie's friendship is when we compare them to others at the ranch .Crooks simply puts it:
It's just being with another guy. That's all.
In the book George and Lennie have a dependent/companion like relationship. They bother needed one another to keep sane and avoid lonliness.
"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. We don't have to sit in no bar room blowin' in our jack jus' because we got no place else to go. If them other guys gets in jail they can rot for all anybody gives a damn. But not us."
- John Steinbeck, Of Mice and Men, Ch. 1
George talks a lot about the relationship between him and Lennie with Slim in the beginning of Chapter 3. These are just a few.
"When his Aunt Clara died, Lennie just come along with me out workin'. Got kinda used to each other after awhile" (Steinbeck, 40).