Since God created spiritual beings for the purpose of expressing love, those beings must have complete free will in order to express that love. Of course, free will allows for the possibility of those beings rejecting God and His plans.
Unfortunately, most people do not agree that God's rules are good and do not want to live by them. In fact, the vast majority of people want to run their own lives without submitting to God at all.
Somehow, they feel that God will just let them into heaven because they have behaved about as good, and maybe even better, compared to the rest of the people on earth. The problem with this idea is that God does not grade on the curve.
All who enter God's kingdom must be absolutely holy, since no sin is allowed in God's presence. In addition, people must be willing to allow God to prevent them from sinning ever again, since there is no sin in heaven. This means that people must be willing to submit fully to God's will in order to get into heaven. Of course, all humans fall short of God's moral requirements.
Therefore, God has made a provision to erase all sins that we have committed in this life and to perfect us so that we cannot sin in the next life.
That provision for sin is through the sacrifice of God's son, Jesus Christ. Jesus took the punishment that we deserve and gives us the reward that we do not deserve - eternal life. In accepting Jesus as Lord and Savior, all our sins are erased, and Jesus works to change us into His image (if we allow Him to do so).
People prefer hell over complete submission to God
So why doesn't God make everyone into perfect beings and allow them all into heaven? It would actually be more cruel if God were to do this, since many people prefer hell to the alternative (complete submission to God).
All the people who end up going to hell will have done so because they actually prefer hell to being forced into the presence of God for all eternity.
People like to live in their favorite sins and answer to no one else. They know that if they accept Jesus as Lord and Savior that God will want them to change their lives and they might have to give up some of their autonomy.
Conclusion
At this time in history, there is more evidence demonstrating the existence of God than at any time in the past. There are missionaries throughout the world proclaiming the good news of Jesus Christ and the salvation that He offers as a free gift.
Although the gift is "free," it will cost you your sin, selfishness, and pride. Since none of these character traits are allowed into heaven, those who insist upon holding onto these traits cannot enter into heaven, but must be separated from God in a place called hell.
Therefore, those who go to hell do so voluntarily, preferring hell to complete submission to a holy and just God.
