Conversely, those animals, such as cattle, sheep and goats, that are permitted to us are herbivorous, and chew their cud. Thus, much of their time is spent in a state of passivity. Their tamer nature makes them more human and allows them to be domesticated. No food, however, can really give life. It is the will of G-d that maintains us.
Eating represents a challenge, and hence, a tool for growth.
When we eat, it seems to us that food is what sustains us. It is our test to realize that we are really sustained by G-d, who created us to eat in order to challenge us with the ability to see beyond our need for physical nourishment to the fact that He is the source of life. Rabbi Eliyahu Eliezer Dessler, in his book, Strive For Truth, goes so far as to say that it should have been that when a human being eats, he will die, and when he does not eat, he should live. By eating, the person seems to say that food is the source of life. Refraining from eating should nourish us because by this we declare that it is G-d who sustains us.