Explain how the resolution of the plot indicates the author's purpose.

John Steinbeck's novel Of Mice and Men follows George Milton and Lennie Small, two migrant field workers who are traveling through California with the ambition to one day settle down on their own parcel of land. Lennie, who is mentally disabled, wants nothing more than to care for pet rabbits on the land; he has a caring streak in spite of the fact that he is unable to deal effectively with boundaries, which resulted in him being accused of rape after touching a young woman's dress and refusing to let go back in Weed. George largely serves as Lennie's protector.
Lennie and George are hired at a farm, where the antagonistic Curley (son of The Boss) torments them. The pair deals with a myriad of other issues, including the fact that Curley's wife is quite persistent in flirting with Lennie. In spite of these issues, the prospect of owning land appears tantalizingly close when Candy, the elderly handyman, offers to pitch in the $350 he has saved in order to buy the land.
Lennie's uncontrolled strength is revealed through a series of violent events: he crushes Curley's fist after Curley attacks him, he accidentally kills the puppy he was given by Slim, and then he accidentally kills Curley's wife by breaking her neck while stroking her hair. 
In the tragic resolution of the novel, George must make the difficult choice to shoot Lennie at their old camping spot. He knows that Lennie cannot possibly escape from the men who are now pursuing him for Curley's wife's death, and he also knows that his dream of owning a farm is dead. 
This resolution emphasizes one the major themes/purposes of the book: to highlight the trials and tribulations of being a human. Throughout the book, Steinbeck uses the obstacles that Lennie and George face to indicate how difficult it is to pursue a dream. What is deeply ironic about the novel's resolution—in which George shoots Lennie—is that George is finally confronted with the truth that the very thing that made the pursuit of the farm worthwhile (his friendship with Lennie), is what stood in the way of the dream of the farm being actualized.