from this link Basically, the reason Jesus had to die for our sins was so that we could be forgiven and go to be with the Lord. Jesus is God in flesh (John 1:1,14; Col. 2:9) and only God can satisfy the Law requirements of a perfect life and perfect sacrifice that cleanses us of our sins.
All people have sinned against God. But, God is infinitely holy and righteous. He must punish the sinner, the Law- breaker. If He didn't, then His law is not Law for there is no law that is a law without a punishment. The punishment for breaking the Law is death, separation from God. Therefore, we sinners need a way to escape the righteous judgment of God. Since we are stained by sin and cannot keep the Law of God, then the only one who could do what we cannot is God Himself. That is why Jesus is God in flesh. He is both divine and human. He was made under the Law (Gal. 4:4) and He fulfilled it perfectly. Therefore, His sacrifice to God the Father on our behalf is of infinite value and is sufficient to cleanse all people from their sins and undo the offense to God.
Here is another:
link to source When Jesus died, He died for a reason. His death was not an accident. He came to this earth in order to forgive our sins. He had to die in order to do this. He came because Adam and Eve had disobeyed God.
Sin is disobeying God. God had warned our parents - Adam and Eve - that disobeying Him would bring death. They would die both physically and spiritually. Spiritual death is eternal separation from God - not destruction of the spirit. We understand physical death. When Adam and Eve sinned they caused their children to become sinners. They died both physically and spiritually.
Questions for debate:
1. Did Jesus die in order that we would be forgiven for OUR own sins, for the sins of Adam and Eve, or both?
2. If this was to forgive us for the sins of Adam and Eve, why did WE need to be forgiven for the sins of our ancestors? Was God being unjust and vengeful to punish the innocent descendants of this sinful pair?
3. If Adam and Eve's sin caused their descendants to be sinners, are we ourselves truly to blame?
4. Why was forgiveness contingent on punishment? i.e. why couldn't God simply grant amnesty for the sins? Is God simply being legalistic - being bound by the very laws that he himself created?
5. If punishment was indeed necessary, how does it make sense that ANOTHER person (Jesus) can take the punishment that leads to forgiveness?
6. If Jesus is God, part of the Trinity, then how can his suffering truly be any kind of sacrifice? He had to endure a day or two of pain, with full knowledge he'd again be walking around 3 days after dying. What is the big deal here? This is trivial compared to the suffering of a cancer patient who suffers for months or years, and then dies - only to remain dead. (the fact that, in theory, the cancer patient goes to heaven is irrelevant...Jesus did that as well - only he got to take his earthly body with him).