The question of why Jesus died, has everything to do with who Jesus is. Jesus asked the question, "Who do you say that I am?" It was the demons that were the first to recognize Jesus. When Jesus went to the country of the Gerasenes to heal a man who was demon possessed, upon seeing Jesus, the wicked spirits that had possessed the man cried out, "What business do we have with each other, Jesus Son of the Most High God?" (Mark 5:7).
When Jesus asked his disciples, "Who do men say that I am?" they responded, "John the Baptist, Elijah, Jeremiah, or one of the prophets." People in Jesus day were confused about who Jesus was. But when Jesus asked his disciples, "But who do you say that I am?" Simon Peter replied,"You are the Christ, the Son of the Living God." Peter knew exactly who Jesus was, and Jesus said to him, "Blessed are you Simon Barjona, because flesh and blood did not reveal this to you, but my Father who is in heaven" (Mat. 16: 13-17). However, after Jesus told his disciples that the Christ must suffer, be killed, and raised on the third day, Peter rebuked him, saying, "God forbid, Lord. This shall never happen to you."
What Peter, and the disciples did not understand, is that Christ, Messiah, and Son of the Living God, includes suffering, death, and resurrection.
But why did Jesus have to die?
God in Christ entered into our rejection of him. He vetoed our no to him. He died our death to destroy sin, and its consequences: pain, suffering, sickness, and death. God united himself to humanity, in the incarnation, in the person of Jesus, and when he died, we died. When he was raised from the dead, we were raised with him. When he ascended to heaven, we ascended with him, and we are seated with him, in heavenly places (see Gal. 2:20; Eph. 2:5-6; Col. 3:3-4; 1 Cor. 15:3, 13-22, 26, 42-57).
By his death, Jesus put an end to the Old Covenant, having fulfilled it completely. He established a better covenant, based on the promise of a new life (Heb 8). Jesus is the pioneer of this new life. Being the Godman, Jesus has fulfilled both sides of the covenant for us, and as us. As God, he says yes to humanity, and as man, he says yes to God.
The old life in Adam is null and void. We now live in the resurrected life of Christ, participating in the trinitarian life of God, by his grace.
Watch my video, "Why Did Jesus Die?"