This, in many ways, is the most difficult question for Christians. Probably the hardest thing for people to comprehend is what the Bible has to say about the destiny of the wicked - they will be judged and suffer for rejecting Christ. This concept of judgment is not hard to understand. What is difficult for people is the length of punishment - everlasting. Why should there be no end to someone's suffering? How could a God of love allow this?
Made For Eternity
We stress the fact that humans are made to exist eternally. The Bible says that each soul, or spirit, will exist somewhere forever. There is no such thing as the extinction of the soul.
Holy
Part of the problem is not recognizing the awfulness of sin. Humanity has sinned against a holy God. Because God is holy sin requires judgment. This is the area that most humans do not appreciate - the awfulness of sin and the holiness of God. Our sins have offended a holy God and must be paid for. Either we will let Christ pay for them or we will pay for them ourselves. One way or another, our sins have to be paid for.
Degrees Of Suffering
Suffering will not all be the same. Scripture teaches that there will be degrees of suffering in hell. Not everyone will be punished to the same degree. People will be punished based upon the amount of truth which they have received and rejected.
Summary
Human beings have been made for eternity. Since people have sinned against a holy God they must be judged for their sin. The Bible says the length of their suffering is everlasting. This is because each of us has been made for eternity. Though all will be eternally punished, not all will be punished to the same degree.