Survival is the entire point and focus of Pi's existence. It is what he thinks about every day, all day. He has to figure out how to survive or else he will die a slow and painful death on the seas. For Richard Parker, survival isn't as much of his focus, since he doesn't have the higher reasoning skills or a perception of the danger of the situation. So, Pi is left with the stress, responsibility and struggle of hacking out surivival for both of them, for months on end.
From morning until night, Pi has to ask himself, "What am I going to do today to keep us alive?" He has to think about how to keep them in a supply of food and water, and how to keep himself from getting depressed or discouraged, or from succumbing to the elements.
Such an arduous struggle for survival can really strain the human soul. Pi is depressed and hopeless; he struggles with questions of why, and of his existence. He ponders his own nature constantly, berating himself for past mistakes and trying to give himself pep talks. He loses some aspects that were previously characteristic to him--his optimism, his open mind, and his values of vegetarianism and moral adherence. His nature is strained; the more animal-like traits come out. He acts like an animal when it comes to eating, maintaining his dominant status on the lifeboat, and even when associating with others. His nature is reduced and stripped to its most basic and animalistic tendencies, all centered on survival.
I hope those thoughts helped; good luck!