Ephesians 2:1-3 explains that we are born "dead in our trespasses and sins." This sin causes us to be separated from God. The One, True, Holy God cannot have a relationship with us, when we are in sin. And there is nothing that we can do to work our way out of our sinful state.
Romans 3:23 says, "For all have sinned and fallen short of the Glory of God." Romans 3:10 explains, "There is none righteous, no not one." And Romans 6:23 concludes that "the wages of sin is death." You see, because of our sin, the just and right punishment is that of eternal damnation.