Which qualities do George and Lennie bring out in each other and how do they do this in the novella Of Mice and Men?

George and Lennie have an almost older/younger brotherly relationship, and there is a psychological dependency on both their parts.

In the opening scene of Steinbeck's novella, George and Lennie enter the clearing where they will camp for the night before going to work at the ranch the next day. While they are there, George has to scold Lennie to not drink too much water when they find the pond; then, he cautions him again to avoid water if it is not running. Later, in a childish fashion Lennie complains that he wants ketchup on his beans despite George's insisting that he has none. George must scold Lennie for playing with a mouse; in disgust, he tells Lennie that he could get along well without Lennie, and he could even have a girlfriend. But, when Lennie threatens to run off, in brotherly fashion, George confesses, 

"I want you to stay with me, Lennie....somebody'd shoot you for a coyote if you was by yourself."

While it is merely a pipe dream, there is much truth in George and Lennie's recitation of their hopes of owning a farm. When George recites how men like them have "nothing to look ahead to," Lennie becomes delighted, asking George to continue.

"With us it ain't like that. We got a future. We got somebody to talk to that gives a damn about us...."

Lennie breaks in, "But not us! An' why? Because...because I got you to look after me, and you got me to look after you, and that's why."

After a while, George confesses that with Lennie he has begun to believe in the dream. Just as Crooks speaks of the need of a man to have someone else by whom "to measure" himself, George and Lennie have each other, a fraternity, that gives their lives some meaning. While George tries to protect the child-like Lennie and offers him friendship. Lennie provides George companionship, meaning in life, and affection. All this is expressed in their recitation of their dream.

Robert Browning once wrote,

Ah, but a man's reach should exceed his grasp,
Or what's a heaven for?

George and Lennie give each other a grasp of something beyond the next ranch, the next job, the next meal. They give each other some hope during a time of great despair in America; they provide each other comfort in a comfortless world.