Why do living things die?

A living organism is in a constant state of breakdown (injury, infection, etc.), balanced by a constant state of repair and/or growing. Eventually, the ability of an organism to maintain all the necessary functions and structures fails. When it fails, it dies. Numerous theories abound as to why an organism can not sustain itself indefinately. Some are obvious, like an injury too severe to be repaired by the body. Others like different lifespans and disease are affected by many factors. Death also gives life it's significance.