death why?.................. a tree lived for .3,500 years until it was burnt down

1  if deerghaayushya is possible, amruthathva also is possible as a corollary  

jathasya maranam dhruvam .......need not be true always ,  can be defied

one need not die, death is not inevitable, 

2   then how man dies??

man dies because, ''wages of sin is death ''

if sin not done, then death wont be 

3 why / how sinning happens??

man sins due to ignorance 

4 what is sin??

sin is to violate the swadharma, by own doing [ or others causing it, ]  knowingly / unknowingly , manly due to not knowing how to be sinless, due to not knowing how to overcome the effect of sin, ...paapa parihaara

5 why man remains ignorant??

he does not know the value of life, 

so does not find ways and means to preserve/protect life of self and life of others 

due to negligence, will fully or forcibly he yields to death