The reason why so many consider Herman Melville's Moby Dick to be one of the greatest American novels of all time is the reason that it can actually be interpreted in many ways, which is mainly due to the fact that it has multiple themes. For example, it describes the relationship between nature and humans, the meaning of religion, the question of fate and destiny, the pursuit of knowledge, and many more significant themes. However, one of the main themes of the novel is actually the complexity of human nature and human emotion.
The main reason why the the main plot happens is because of Ahab’s hatred for the white whale and his obsession to find it. The captain lost his leg when he and his crew went whale hunting, and he unsuccessfully tried to attack Moby Dick with a knife; thus, he swore revenge against the whale. His obsessive need to find the creature and kill it, or to "dismember his dismemberer," becomes his life mission, and that thirst for revenge drives both him and the action forward.
Captain Ahab is so blinded by revenge that he basically becomes the personification of that emotion and lets it consume him completely; every decision and choice that he makes is influenced by his obsessive need to find the beast and punish it for what it did to him. All other emotions basically become secondary.
Small reason was there to doubt, then, that ever since that almost fatal encounter, Ahab had cherished a wild vindictiveness against the whale, all the more fell for that in his frantic morbidness, he at last came to identify with him, not only all his bodily woes, but all his intellectual and spiritual exasperation.
In the end, Ahab actually realizes how his hatred has blinded him, but it is unfortunately too late, and he pays the price; his need for vengeance and obsession kill both him and his crew.
What is it, what nameless, inscrutable, unearthly thing is it; what cozening, hidden lord and master, and cruel, remorseless emperor commands me; that against all natural lovings and longings, I so keep pushing, and crowding, and jamming myself on all the time; recklessly making me ready to do what in my own proper, natural heart, I durst not so much as dare? Is Ahab, Ahab?
In contrast, Captain Boomer is someone who lost an arm, but instead of filling his life with hatred and negativity, he focuses on positivity and is thankful that he survived the encounter with Moby Dick and got to live another day.
Melville's idea, therefore, is to showcase the depth of human nature and the mysterious ways in which the human mind operates—to describe how people let their emotions or feelings get the better of them, both consciously and unconsciously.
Sometimes, we are willing to spend a lifetime pursuing something that we think will bring us peace or satisfaction, even if we know that such pursuit might be irrational or even futile. Sometimes we let those emotions become us, not realizing how that might affect us or those around us, and we let it push us to the brink of madness, like Captain Ahab did. Sometimes, however, we learn how to cope, and we use our experiences as a source of knowledge—something that can helps us overcome life's challenges more easily and makes us aware of life's beauty and meaning, which is essentially what Captain Boomer did. In each scenario, we define ourselves and our nature and prove our complexity as human beings.