"Maybe if i give you guys money, you'll let me hoe in the garden"
"An' put some grass to the rabbits"
"He ain't gonna let me tend no rabbits"
George and Lennie are talking about living together as they're sitting in the woods. George and Lennie are tired of working, so they dream of living together on a farm with rabbits, which symbolizes the American dream. Dreams are available to everyone, and they make life worth living.
Candy secretly joins in on George and Lennie's dream, as they're in the bunkhouse. Candy is willing to pay for most of the property, and George and Lennie's dream almost appears to becoming true. Though dreams might seem achievable, something always goes wrong.
Lennie kills both Curley's wife and his new puppy, and Lennie notices that he might've ruined his dream of tending the rabbits. As the rest of the workers find Curley's wife dead, George and Candy know that their dream is indeed gone. This is the climax of the story and at this point every character in the book realizes their dreams are ruined. Dreams never seem to go as planned, and eventually dreams are ruined.
George shoots Lennie to death at the brush. George knows that Lennie is going to get killed, so he kills Lennie in the most humane way. It is also right for George to kill Lennie, since Lennie is George's responsibility. This corresponds to the situation with Candy because he was responsible for his dog, but he allowed Carlson to kill his dog for him. This foreshadows this scene in the book, because George doesn't want to make the same mistake as Candy.