Its simply a mechanism for self preservation - just like why you get out of the way of a bus.
If living forever is a natural thing - why is NOTHING built to last forever - including the universe?
Why are we so poorly built? Just about anything can kill us. 71% of the earth's surface could kill you in about 4 minutes (oceans). You can't be outside without protection on about 70% of the rest of the earth's surface without dying from exposure in a matter of a few days at most. I could go on and on and on.