Death is the ending of physical life. As the body decomposes, nobody knows truly what happens after death, except for the decomposition of our anatomy. We probably fear death because life is all we know. We fear for our life to end, as we instinctively sense that death might be the end to our existence. Life after death? Who knows? Physiologically, our bodies strive to continue living at all costs, as this is simply instinctive behavior to survival.
This explains the ending to our existence, but what is the purpose to any existence? Why does life continue? Why are we here? Immanuel Kant called it the noumena; what we don't know yet we are aware.