Ageing is a normal process of human development that takes place on several levels: biological, psychological and social although it is not certain what causes ageing, most gerontologists would agree that ageing is the result of a combination of both internal and external processes.  The  "wear-and-tear theory"   compares the human body to a machine that over time wears down from use. According to this theory,  bodily systems receive cumulative damage from both external forces, such as toxins, released as a result of metabolism. Cells become damaged and increasingly fail to reproduce or repair themselves.  They die off in larger numbers as we age. the  "combination theory" explains that influences such as stress and diet,  each person is born with a genetically predetermined life expectancy that cannot be exceeded  This interaction of external factors and internal programming  would account for individual variations in the life span. The mystery surrounding why we age is still a topic of numerous ongoing studies. Perhaps, one day we will truly  know why we age.