Question

why a person die?what is the scientific reason behind it

Answer 1

Its because of many factors which plays an important role in the life
But if you are considering an ideal condition the reason is due TO AGEING
ageing is the cause of death, ageing causes terminlization of DNA bonding from the telomeric ends .. thus making the DNA regeneration and other viable factors slow and steady .. which then leads to ageing .. that's how we die