This isn't precisely known, but one major reason is telomere depletion.

Every time a cell divides, it cuts off a little bit of its DNA. When a person is conceived (i.e. sperm meets egg), the person has a limited amount of cushion of DNA on the end of their chromosomes, which gets eaten away with each division. Thus, a given cell line can only undergo a certain number of divisions before it destroys its genome, killing the cell and its descendants. Now, reproductive cells regenerate this cushion, but body cells don't. Thus, you can only live as long as your cells can.