Why did Jesus have to die on the cross??

Because of sin, all men are destined for eternal death.  When God put Adam in the garden of Eden, he told him that eating of the tree of knowledge of good and evil would lead to death.

Genesis 2:15 - "The Lord God took the man and put him in the Garden of Eden to work it and take care of it. 16 And the Lord God commanded the man, “You are free to eat from any tree in the garden; 17 but you must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die.”

The words "You will certainly die," here can be literally translated, "you will die in your dying."  This certainly denotes eternal death or at least a second death, or spiritual death as well as physical death."

All men are destined for death because of Adam's choice to eat of the tree of knowledge of good and evil.  Furthermore, all men are captive to Satan, due to Adam's choice.  When God created man, he created an established order.  God directed the affairs of man and man looked to God for direction.  When Adam listed to Satan, he gave Satan a foothold into the human race.  Satan could now interfere with the God-Man relationship.  To simply fogive man would not be enough.  Man would have to not only be forgiven, but redeemed.  Satan now owned the eternal destiny of all men.  God would have to buy back this eternal destiny.

In order to redeem mankind, an infinite price would have to be paid to redeem the infinite sould of every man.  

Jesus was that infinite price.

John 1:18 "No one has ever seen God, but the one and only Son, who is himself God and[b] is in closest relationship with the Father, has made him known."

Collosians 2:9 "For in Christ all the fullness of the Deity lives in bodily form,"

Jesus paid that infinite price by going to hell withh his infinite person.

Ephesians 4:7 "But to each one of us grace has been given as Christ apportioned it. 8 This is why it[a] says:

“When he ascended on high,
    he took many captives
    and gave gifts to his people.”

9 (What does “he ascended” mean except that he also descended to the lower, earthly regions[c]? 10 He who descended is the very one who ascended higher than all the heavens, in order to fill the whole universe.)"

Jesus was so infinite that even the span of hell for all time could not hold him.

Acts 2:24 "But God raised him from the dead, freeing him from the agony of death, because it was impossible for death to keep its hold on him."

The death of Jesus was the gift from God to save all manking from sin and the consequence of sin.

1 Peter 2:24 “He himself bore our sins” in his body on the cross, so that we might die to sins and live for righteousness; “by his wounds you have been healed.”

Galatians 3:13 "Christ redeemed us from the curse of the law by becoming a curse for us, for it is written: “Cursed is everyone who is hung on a pole.”[h] 14 He redeemed us in order that the blessing given to Abraham might come to the Gentiles through Christ Jesus, so that by faith we might receive the promise of the Spirit."

This is why we must believe in Jesus.  It is not enough to just do good.  By one act of eating the fruit of the tree of knowledge of good and evil, Adam placed the whole race of men under the influence of Satan.  Even one thought of covetousness separates us from God and destines us to eternal death.  Jesus' death on the cross is God's gift to us.  To receive it, we must believe it, and believe in Jesus.