A simple answer is Living things regenerate their cells continuously, So they can maintain healthy function. When a living organism can not regenerate cells then they begin to degenerate and that leads to death. When Organic life dies it begins to decompose because the cells die. The individual cells can continue to live for several days after a organic organism or a person dies. But eventually the cells brake down and die. So the body decomposes. In certain Conditions organism's can live for thousands of years After the entity dies, But this is rare.
