Sin causes death.
Ever wondered why people die? Sin, that's the reason.
Through Adam and Eve, sin entered the human world. Because of this, death became a reality for all people.
12 Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all people, because all sinned—
(Romans 5:12 NIV)
Death is the reward for sin.
23 For the wages of sin is death ...
(Romans 6:23a NASB)
In the Garden of Eden, Adam and Eve were able to eat from the tree of life. Scripture indicates that, as the name indicates, the tree of life provides food that heals the human body.
When GOD gave Adam and Eve the (only!) command to avoid touching and eating of the tree of knowledge of good and evil, GOD warned them that, if they disobeyed, death would be the result.
15 Then the Lord God took the man He formed and put him in the garden to tend and keep it. 16 And the Lord God commanded Adam, saying, “You may eat food from every tree in the garden; 17 but from the tree of the knowledge of good and evil you may not eat; for in whatever day you eat from it, you shall die by death.”
(Genesis 2:15-17 OSB)
After they ate the forbidden fruit, GOD removed their access to the tree of life:
22 Then the Lord God said, “Behold, the man has become like one of Us, to know good and evil. Now, lest he put out his hand and take also of the tree of life, and eat, and live forever—” 23 therefore the Lord God sent him out of the garden of pleasure to cultivate the ground from which he was taken. 24 So He cast out Adam, and made him dwell opposite the garden of pleasure. He then stationed the cherubim and the fiery sword which turns every way to guard the way to the tree of life.
(Genesis 3:22-24 OSB)
As we previously established, sin separates us from GOD, breaking our relationship with Him because we have been stained by sin.
Since GOD is life (the eternal self-existent source of life), the moment we become separated from GOD, we begin to die. We immediately experience spiritual death which is the spiritual relational severance. And, like a flower cut from its roots, our body begins to die physically.
Writing to Christians in Ephesus, the apostle Paul explained their former state as:
1 And you were dead in your trespasses and sins, 2 in which you formerly walked according to the course of this world ...
(Ephesians 2:1-2a NKJV)
Paul summarized the human condition in this way:
12 Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned—
(Romans 5:12 NKJV)