The reson for the death of all living things is not clear. However, what is clear is that death is an important part of life and it is the last step in the circle of life. Ever since the date of brith, living things start to use all ways to survive but not to die. They compete with each other for food and other resources that can make them live longer. The reason for the death for individual of the living things can be different. For example, human beings die because of serious illnesses, accidents and so on. Death is just a circle of life and all living things has to undergo the circle of life.

Every living thing know to exist has so far been known to die or at least replace most of its cells. I don't think it is fully understood why some living things age and die faster then others. But living lines are made up of living cells that often die faster than the organisms as a whole because the organisms can replace some of the cells as they age and die. But typically, an organisms ability to replace the cells deteriorates with time so that eventually the whole organism dies.

There is something in physics called "entropy", which is a measure of the disorder in something. One of the laws of physics (the second law of thermodynamics) states that entropy is indestructible: you can move it, you can make more of it, but you can never get rid of it other than by putting it somewhere else.

DNA (the blueprints for our cells) can only handle so many replications because we lose info every time, telomeres get shorter, but there is theoretical research in medicine to preserve telomere length.

Everything wears out as it gets older. When living things get too worn out to stay alive, they die.