That was not our original design. We were originally created to live forever by God's design in the Garden of Eden, but becasue we rebelled against the Word of God (sin), we were cast from paradise and placed under the "curse" that corrupted our physical state of being as well.
With our immortality having been lost and the world (physical universe) having been likewise placed under this "curse"; then... time, decay, aging and death began to occur in the universe. The maximum human lifespan, as designated by the "Word of God" (Genesis 6:3), then diminished with each successive generation thereafter, from as much as almost 1000 years (Methuselah) at old age (Genesis 5:27) to the 120 years that Moses lived before he passed.
Thus is the "curse" that our physical mortal bodies are now subject to. But, our soul and spiritual bodies are still eternal and the Lord God has a plan however, for us to return our physical bodies to an immortal state once again. This will reunite soul-spirit-body together in an eternal state of being. That plan is in our salvation from sin (John 3:16-21). There is a Kingdom Coming to this World (Rev.20+21) when the Lord returns and all who believe in Him will be ressurrected into these perfect glorified bodies once again (Rev.14+20). Those who have not been "purified" will suffer not only physical death but also spiritual death. Those who have accepted God's forgiveness shall be spared both deaths and live forever with Him (Rev.22).
I pray that this helps to to reveal an insight to many that will spare them this future fate. DEATH IS NOT BY DESIGN, BUT OUR CHOICE as default of not accepting eternal life. WE ARE BORN TO LIVE.