God gave us a choice. We can choose to love ourselves and turn to our selfish pursuits. That is sin. In our sin we cannot know God and His love. The just payment for our sin is death.
Death here means spiritual insensitivity.
When we are still in our sin, we have no life with God. We are alive physically but dead spiritually. If we continue in that condition, we will be separated from God for all eternity. The wages of sin is death. Because Jesus, His Son, has paid the price for sin by His death on the cross.