As critical and close readers will discover, the relationship between the intelligent, but small and weaker George Milton and the mentally handicapped, but large and stronger Lennie Small [notice the last names!] is symbiotic, just as the second post clarifies. Even Lenny expresses this relationship:
because I got you to look after me and you got me to look after you, and that's why
While George complains in the first section of Steinbeck's novella Of Mice and Men that he does not know why he keeps Lennie around--"I could get along so easy and so nice if I didn't have you on my tail"--he later explains the importance of Lennie's friendship:
I ain't got no people....I seen the guys that go around on the ranches alone. That ain't no good. They dont' have no fun. After a long time they get mean. They get wantin' to fight all the time.
So, although he is often irked with Lennie, George stays with Lennie out of commitment to his promise to Lennie's aunt, but also because he cares about Lennie and understands that, as Joseph Conrad wrote, "Meaning in life depends upon sharing." This idea is central to Steinbeck's theme of the brotherhood of man and how men fare better if they are not alienated because, if alienated, they become mean and cruel out of their fear of vulnerability, as does the character, Curley. The strength to oppress others is itself born of weakness, Steinbeck tells his readers.
Most importantly for both George and Lennie, Lennie is the keeper of the dream. For, without the child-like Lennie there is no dream of a ranch and rabbits and "livin' off the fat of the land." It is for Lennie's sake that George repeats the dream of their having land; George does not really believe that this dream will come to fruition--at least, at first. But, with his childlike friend's belief being so strong, George himself starts to believe in a future, thus acquiring some hope in his life. So, Lennie not only gives George much needed love, but he also lifts George from his life "of quiet desperation" as Thoreau wrote.
When Lennie dies, so does the dream for George and old Candy. And, this is why George has Lennie recite the dream one more time before his death:
George shivered and looked at the gun, and then he threw it from him, back up on the bank, near the pile of old ashes. [symbol of death of Lennie and the dream]
According to Steinbeck, all men need someone to help them measure the world. The dysfunctions and psychological complexes of the other characters all stem from their lacking this someone. After all, it is the predatory human tendencies that defeat Lennie and George in the desperate mouse maze of life of the Great Depression.