We die naturally. Our bodies become physically incapable to work at a certain age, usually around 70 years old. Some people will have their bodies shut down at an earlier age, some people later. Sicknesses play a huge part in death, too. We die usually of old age, but other reasons like murder and sickness can lead to death also. There are other natural (but abnormal) causes of death, too, like cancer. Around 20% of people develop cancer - although not all of them die because of it. Smoking and drinking is another reason why people die.
A religious view is that God has made us all to see what we will do with our lives. We will then die and see if we are worthy to enter into heaven. So no death is without a purpose. People die because they are meant to, by God's decision, even if it seems an unexpected or untimely death.