I think that one can see that Steinbeck makes it clear from the earliest of points that there are external factors that will converge and conspire to make it so that the dream that both Lennie and George share will be denied. For example, consider that nearly every character remarks to them that their dream is one that has been shared before and never to be realized. At the same time, Steinbeck develops Lennie's character as one that is rooted in complete fear when he has to fight with Curley, almost to the point where he is terrified that his dream of taking care of the animals will never come true. The same experience is felt when Lennie interacts with Crooks as well as Curley's wife. The overall feel of the novella is one in which the external conditions are almost rigged so that their dream will never be realized. Steinbeck's inclusion of Candy's dog's shooting as well as how George and Lennie are never able to enjoy an opportunity of luck or positive circumstance that enables their dream to mature helps to make the case rather clear that they are battling the elements. It is almost a foregone conclusion when Lennie takes a hold of Curley's wife's hair that their dream will never be able to be realized.