Logical and scientific answer:
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. (found
link)
Theoretical answer:
Nothing lives forever. ^^