Very simply, as Adam’s children, we inherit death in our bodies. We are born sinners. “Look, I was guilty of sin from birth, a sinner the moment my mother conceived me.” Psalm 51:5 (NET)

Adam could have lived forever, but as we know, he chose to disobey. God forewarned him that “Of every tree of the garden, you may freely eat, but of the tree of the knowledge of good and evil, you shall not eat of it, for in the day you eat thereof (dying) you shalt surely die.” Genesis 2:16, 17. 

Happily, God provided a solution to death through Jesus's sacrifice. “For as in Adam, all die, even so in Christ shall all be made alive.” 1 Corinthians 15:22