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. 4:5-6) 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.
from: http://www.carm.org/doctrine/Jesusdieforsins.htm
