This is a great point. One of the main characteristics of Lennie and George is that they are good friends. This is in stark contrast to all of the other characters. Everyone else is alone. For example, the men in the bunker are merely acquaintances. No one likes Curley, even though he is the boss's son. His wife also despises him, and she feels alone.
Candy, too, is alone. He feels as though he cannot contribute much to anyone. And arguably the one who is the loneliest is Crooks, the lone black man on the ranch.
As for Lennie and George, they have each other. Here is the best quote; it is a dialogue between the two men.
Lennie said craftily—“Tell me like you done before."
“Tell you what?"
“’Bout the other guys an’ about us."
George said, “Guys like us got no fambly. They make a little stake an’ then they blow it in. They ain’t got nobody in the worl’ that gives a hoot in hell about ‘em—"
“But not us,” Lennie cried happily. “Tell about us now.” George was quiet for a moment. “But not us,” he said. “Because—”
“Because I got you an’—
“An’ I got you. We got each other, that’s what, that gives a hoot in hell about us,” Lennie cried in triumph.
Here is another quote that shows that Lennie and George are friend. In a conversation between George and Slim, George says that they take care of each other. Slim is surprised, because usually men travel alone.
“You guys travel around together?” His tone was friendly. It invited confidence without demanding it.
“Sure,” said George. “We kinda look after each other.” He indicated Lennie with his thumb. “He ain’t bright. Hell of a good worker, though. Hell of a nice fella, but he ain’t bright. I’ve knew him for a long time."
Slim looked through George and beyond him. “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.”
In the world of "Of Mice and Men," friendships are unusual. What Lennie and George have is rare and precious.