Your inquiry is actually a philosophical one, involving questions of purpose, reason, cause-and-effect, and existentialism, as well as physiologically. “Why” is a solely human concept; there is no evidence that any other creature asks “why” about their existence. As Walt Whitman says, “Animals do not lie awake in the night and weep for their sins.” Most cultures begin with the assumption that we exist for a purpose; Christianity, for example says “Who made you? God made you. Why did God make you? To love, honor and obey Him.” Existentialism refutes the question, and gives more importance to free will and human choice. Physiology simply speaks of energy gathered and spent, the “great chain of being,” survival of the fittest, and such natural explanations. Behind your question is a respect for all living things, and a challenge to the assumption that because Homo Sapiens has evolved to a high degree, we humans have dominion over other life forms. Some religions ask us to modify that assumption by refusing to eat or even harm other animals, even insects. A vegetarian gets all his or her nourishment from plants. But suppose I objected to eating “wheat,” or "rice" or "apples" on the grounds that wheat, too, was alive, and deserved to be left to live? In most modern civilizations, our dominance over the animals is assumed, but of course not all. All sorts of “logical” excuses are put forth, and words like “soul” are used; perhaps mammals are more “important than” “fish” because higher on the evolutionary scale, etc. But in the final analysis, we do what gives us the result we want. If satisfaction of hunger is more important than, say, social approval, we choose it.