Why do people have to die? When you think about it it can make people mad at God or it will drive them away. Why does God let suffering happen? Suffering is hard for people.
To understand why people die, we have to go back to when death first appeared in the world. God created a perfect world and placed Adam and Eve in a garden filled with every type of flowering tree and fruit tree (Genesis 2:9). In the middle of the garden, God placed one tree which Adam and Eve were told not to eat of its fruit. You might wonder why God did this. But if you think about it, you will understand that God wanted people to serve Him because they choose to do so. If people were never given a choice, then you might as well say they were forced to serve God. God made the choice easy. There were plenty of other trees in the garden. There was no need eat the fruit of this one particular tree.
Yet, despite the warning "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, for in the day that you eat of it you shall surely die" (Genesis 2:16-17), Adam and Eve ate of the forbidden fruit anyway. As a consequence, death entered into the world. "In the sweat of your face you shall eat bread till you return to the ground, for out of it you were taken; for dust you are, and to dust you shall return" (Genesis 3:19). Because man sinned, he was denied access to the tree of life. God declared that is not proper for sinners to live eternally (Genesis 3:22-24).
Therein then lies a great problem, "For the wages of sin is death" (Romans 6:23), and all of us commit sin "for all have sinned and fall short of the glory of God" (Romans 3:23). Even though our sins are not like Adam and Eve's sin of eating the forbidden fruit, we still sin in other ways. "Therefore, just as through one man sin entered the world, and death through sin, and thus death spread to all men, because all sinned-- For until the law sin was in the world, but sin is not imputed when there is no law. Nevertheless death reigned from Adam to Moses, even over those who had not sinned according to the likeness of the transgression of Adam, who is a type of Him who was to come" (Romans 5:12-14). Hence, the main reason people die is because people sin. Even those who are innocent of sin, such as small children, may die because others have sinned. Mankind's problem is that once the doors to sin and death were opened, they could not be closed again by man.
But there is another reason we die as well. Though this world started out very good in God's sight, sin has corrupted it. It is decaying. "You, LORD, in the beginning laid the foundation of the earth, And the heavens are the work of Your hands. They will perish, but You remain; And they will all grow old like a garment; Like a cloak You will fold them up, And they will be changed. But You are the same, And Your years will not fail" (Hebrews 1:10-12). Perhaps you have a favorite pair of jeans that you like to wear. They are very comfortable and fit you well. But one day those jeans are destined for the trash can, because they will become so worn that they are no longer fit to wear. In the same way this world is wearing out.
Worse, each of us wear out as well. Our body do not last all that long in this world. "But we have this treasure in earthen vessels, that the excellence of the power may be of God and not of us. We are hard pressed on every side, yet not crushed; we are perplexed, but not in despair; persecuted, but not forsaken; struck down, but not destroyed-- always carrying about in the body the dying of the Lord Jesus, that the life of Jesus also may be manifested in our body. For we who live are always delivered to death for Jesus' sake, that the life of Jesus also may be manifested in our mortal flesh. So then death is working in us, but life in you. ... Therefore we do not lose heart. Even though our outward man is perishing, yet the inward man is being renewed day by day" (II Corinthians 4:7-12, 16). Have you seen how frail very elderly people get? When people think about not dying, they imagine themselves in young, healthy bodies. But we all know that our bodies wear out. Who would want to live forever trapped in a broken body?
In order to trade this body in for a better one, we have to leave the body -- in other words, we must die. To get to a better world where sin and does not exist, we must leave this world where they abide, which again means we must die. "Foolish one, what you sow is not made alive unless it dies. And what you sow, you do not sow that body that shall be, but mere grain--perhaps wheat or some other grain. ... So also is the resurrection of the dead. The body is sown in corruption, it is raised in incorruption. It is sown in dishonor, it is raised in glory. It is sown in weakness, it is raised in power. It is sown a natural body, it is raised a spiritual body. There is a natural body, and there is a spiritual body" (I Corinthians 15:36-37, 42-44).
"He then would have had to suffer often since the foundation of the world; but now, once at the end of the ages, He has appeared to put away sin by the sacrifice of Himself. And as it is appointed for men to die once, but after this the judgment, so Christ was offered once to bear the sins of many. To those who eagerly wait for Him He will appear a second time, apart from sin, for salvation" (Hebrews 9:26-28).