The serious answer is that all living things get old and die, and that bodies wear out. Depressing to think about, though, when it comes to ourselves, or people we know. Although it’s a lot nearer for me than for you, I should think..!
There’s another more complicated answer about ageing (getting older) over here.
i actually really enjoyed the recent series of ‘Torchwood’ that tried to address what would happen if everyone in the world stopped dying.
Thinking about this seriously, to be immortal you would have to devise a way of ensuring that your cells throughout your body could continue to divide as often as they needed to to replace worn out cells. The problem comes later in life when those cells get tired and start making mistakes. Those mistakes sometimes lead to cells growing out of control and becoming cancerous. There are also protective caps on the ends of your chromosomes called telomeres that get shorter and shorter as a cell divides and ages. If you could prevent the above processes then you might be able to live longer.
Generally, most young people die through accidents. Older people tend to die of an accumulation of errors that build up over the course of your lifetime. Humans generally are living longer due to better diets and healthcare. This is why we are seeing increases in degenerative diseases like Alzheimer’s. Before modern times, the average age for an adult was a lot younger, and so we didn’t need to adapt to diseases of old age as we’d be dead before then!
If we ever did become immortal, it would probably be an extremely expensive option that would only be open to the very wealthy. I don’t think that would be fair to the rest of humanity, in addition to which those immortal people would become very lonely as everyone they knew who be dead and gone. Quite a sad and thought provoking question so thank you for asking it.