The friendship between Lennie and George is based on loyalty and a common dream. George agrees to take care of Lennie and assumes responsibility for him as a parent would for a child. George does this because, beneath his rough exterior, he is an admirable man. George and Lennie also rely on, and are loyal to, each other because they share a dream of owning their own farm some day. George reluctantly retells the story of their future farm at Lennie's request; each time Lennie asks for this, George does so begrudgingly but must also enjoy entertaining the dream aloud.
The other significant aspect of their friendship is that it sets them apart from typical ranchers. While most itinerant ranchers move from job to job, saving little money (some spending it on booze and brothels), Lennie and George keep each other honest and out of trouble. This gives them a strategy, based on friendship, that will give them a better shot at saving more money. The two men rely on their friendship to facilitate their dream:
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." (Chapter 1)
In the end, this genuine and strategic friendship does not result in the dream of a farm. This is because Lennie is never quite able to adapt to this lifestyle and because the social world of itinerant ranchers makes it very difficult to save money and rise to a higher class. As strong as the friendship is, the harsh reality of their social situation makes their dream unreachable.