Can someone explain this OF MICE AND MEN statement? 10 POINTS FOR FIRST HELPFUL ANSWER!?

In a revision guide it is talking about the main themes - one of which is 'Human Fragility'. It says "The overriding theme of the novel is that humanity is small and fragile in comparison with the forces that control our lives". What forces do you think it is talking about in relation to the novel, 'Of Mice and Men'. Thank you for any help!