They are responsible for each other. The implication here is that man without hope and love, without a dream, is perhaps better off dead. The concluding pages to the novel find themselves in the same setting as the beginning, where they recite for the final time, their impossible dream which finished when the trigger was pulled. What was done had to be done, and the story concludes when both God and man symbolically forgive the murder when the Godly words of Slim declare "You hadda George, I swear you hadda.".
Man against Himself: This to me clearly relates to how George puts his own needs first one and looses his dream. This is shown rather clearly when George goes into town and leaves Lennie behind to do as he pleases. He is not worried about what kind of trouble Lennie may get in to; all he is thinking about is having the chance to go out with the guys and have a good time. This is something that he could not have done in the past because he had to worry about Lennie and make sure that he was not going to get into any trouble that would endanger himself or George. For some reason he does not seem to worry about this when he makes the decision to leave. Is it because he is happy that Lennie is not going to be around him this time and that he can go and do as he pleases without having to have Lennie tag along and put George in an awkward position with the rest of his fellow workers? It seems that this is exactly what he is thinking when he totally disregards the idea that Lennie could make a mistake which would lead to dire consequences. When George finds out that his neglecting to supervise Lennie has led to a tragedy he never once blames himself. If he would have been watching Lennie and taking care of him as he promised to do, none of this would have happened and nobody would be dead. Now, due to George's lack of loyalty to Lennie and his promise to take care of him, Lennie is doomed to suffer the consequences of an action that could have been prevented had he not been left alone.