Could be because the body grows old no matter how much we try to fight it. Regardless of how strong we may think we are, the body wears and tears under duress. If it takes a century of living to wear-out, then you die of old age, but it could take some seconds of crushing and banging, smoking weed, having aids/cancer to speed up the process, thereby taking younger lives.
As soon as the body gives up the ghost, it is time to meet our maker, whether or not we've planned it.
Some believe the body is soul-less, but it is the soul that has a body. The soul is said to live forever. As soon as the body gives up the ghost, the spirit begins to live on forever.
Perhaps we die for the same reasons we live. Either way, without death, there can be no life. If we don't die, the world would become overpopulated, and then we all die. We're disposable systems that are designed to fail at a specific time. Perhaps we die to let others live.