The short answer is that certain of our cells are designed to divide only a certain number of times.  Those researching DNA believe that each time a cell divides (and its DNA divides) a small portion at the end of the DNA may be lost.  A longer answer would try to explain why we die too soon but that gets into diet, exercise, smoking, drinking, driving too fast, and poor self esteem; but that would take a book.\n\nLive long and be happy.
why do we die?