I would add as well that, in the story, the symbiotic relationship of George and Lennie also suffered the sad incidents caused by Lennie that directly led to them a) having to change what "their dream" was- having enough money to lead an independent life, and b) giving it up when George ended up shooting Lennie to avoid the lynching that was coming to him for accidentally killing Curley's wife.
The conflict in this case is the many ways that George tried so hard to include Lennie in his dreams, basically b/c he had not much of a choice but to take care of him due to his condition as a dependent, and potentially retarded man. This, contrasts with the fact that because he is "stuck" with Lennie he has to suffer the consequences of Lennie's incapabilities, and this led ultimately to the end of it all. It was a no-win situation.
On this, George says:
"Whatever we ain't got, that's what you want. God a'mighty, if I was alone I could live so easy. I could go get a job an' work, an no trouble. No mess at all, and when the end of the month come I could take my fifty bucks and go into town and get whatever I want" (p 11-12).
On losing the dream, George says:
"I think I knowed from the very first. I think I knowed we'd never do her. He usta like to hear about it so much I got to thinking maybe we would" (p 103).