Replying to Biologic reason for our lifespan

so i saw a documentary about aging research and how they were able to manipulate mice dna to have them live longer.

which got me thinking, what is the biologic reason for our lifespan? why is there a biologic need for death?

(assuming that it would be possible for nature to erase the aging process as a whole)

maybe it has something to do with change. living forever means that having offsprings is really problematic since the whole world would eventually be "full".

but then again, why are we living around 80 years (prolly more like 30-50 without modern medicine/food supply)? and not a week or a month or a year or thousand years?

discuss, and i am serious