No one is immortal like God,we all die at some point later on when we age around 70's of a certain age of your system weakening.Others may have a disease which causes them to die passed down by generations but who knows..only few lucky people can still be alive age 100 and beyond that a few.
Because god wanted it that way. People used to live for 100's of years. In time god has changed that for us. I think is because we dont deserve to live for that long, just because within time humans have become worst, so i think he took that privilege from us. But if we wouldn't have death, we wouldn't have a motivation to live and people would be crazier!
Physically, our bodies aren't meant to last forever. Eventually, they either break down or wear out. Religiously, Adam and Eve sinned and got us kicked out of paradise. Of course, we all probably would have done the same thing, since Adam and Eve were the best humans that could possibly be made with free will and had seen God but disobeyed Him, but it's still their fault.
Think of the cycle of life for a second; it is just as much a cycle of death. New things wouldn't be presented into the world if old things didn't attenuate their being. Due to the law of conservation of mass, the energy from the deceased is given systematically to the fresh life. Like if an animal dies, it's decomposed and becomes equivalent to the grass of the field in which it dies. Another animal comes about and eats it, and the energy of the dead belongs to that animal. We die because the future must live.