Lots of things can kill you. Because there are so many varied ways in which we can die, I don’t think we’ll ever be able to stop it. Science has come a long way in being able to treat and prevent illnesses, making us live a lot longer than we ever have.
Whilst our bodies are amazing machines that can regenerate certain cells during our lifetimes, all cells have a limited lifespan. After that the cells stop replacing the ones that die and eventually organs die causing a person to die. Of course there are things that speeds death along such as bullets and knives and poison etc 🙂 Disease also interferes/stops/speeds up the cells ability to replace other dead or damaged cells causing death too.
Will we ever be able to stop death? I hope not. If everyone lived for ever it would get pretty boring after a loooong while.
The cells of our body accumulate damage over time, that leads eg to mutations. Therefore, there is a mechanism of “programmed cell death” by which cells die after a while. There are some cells that don’t have that mechanism and that just live and reproduce forever: Cancer cells.
So, really, that cells die is a kind of protective mechanism, and if they don’t, they cause a whole lot of trouble. I think individuals probably die for similar reasons. The earth would be a very weird place if everyone who ever lived was still around.