Why did God need a Son?  Because He loves us.

  WE  needed Him to do this too.  God is a God of truth and justice, and He knew that there was nothing we could do in our power that would atone for our wrongdoing (sin). Therefore God in his infinite wisdom came to earth in flesh, to be born of Mary, so that all people could be reconciled with him and enter the kingdom of heaven.

"And the angel answered and said to her, 'The Holy Spirit will come upon you, and the power of the Most High will overshadow you; and for that reason the holy offspring shall be called the Son of God.'" Luke 1:35

When Adam disobeyed God and was thrown out of the Garden of Eden, he condemned mankind to a life of inherent sin, and we lost something incredibly precious. Sin (wrongdoing) put a barrier between us and God.

The Jews, God's chosen people, sacrificed a lamb to God, once a year in Jerusalem, to pay the price of their wrongdoing. When the Jews sacrificed their lamb, it had to be a perfect one - not one mark on it. Only the best was good enough. Jesus was the only man who had never sinned. He was the only man who could have paid the blood sacrifice for sin because a sacrifice had to be perfect.
Christ's death on the cross was to pay for all people's wrongdoing.
It is through this spilt blood and belief in Him, that all are now acceptable in the eyes of God, when we repent and accept Christ into our lives. God loves us that much that He chose to have Jesus born on the earth and have him sacrificed for our sin.
Jesus was born of Mary and was conceived by the Holy Spirit. That made God his Father. This is why Christ is referred to as God's son, even though it was God taking human form. This Son of God gave us all he had to offer. He paid the ultimate price and through His grace we are now able to be called children of God. When we hurt, He knows what we are feeling!
Where would we be if he hadn't come?

No one can earn this grace, it is a gift God gave us freely. Not one of us is worthy of receiving it. Not one of us is worthy of the title of brothers in Christ, yet when we accept Christ into our hearts we are adopted into God's family. He marks us as his own. We can't earn it by working hard or good deeds. We have to step out in faith and believe it and receive it.Unfortunately,  we are blinded to this truth until we take the step of faith and acknowledge Him as Christ.

"But whenever a man turns to the Lord a veil is taken away."  2 Cor 3:16

Yes, God is a Just God and a powerful one. The one true God we should all hold in awe. But through His coming to earth we can now call him "Father" too, and have a life-changing personal relationship with Him. He came to complete the Jewish faith, not compete with it. As Abraham was promised

"And in your seed all the nations of the earth shall be blessed, because you have obeyed my voice."
                                                                                                                            Gen 22:18

After Christ went back to heaven he sent the Holy Spirit to be with us. He is our comforter, helper, and intercedes for us when we don't know how to pray:

Romans 8:26-27
                In the same way the Spirit also comes to help us, weak as we are. For we do not
                know how we ought to pray, the Spirit himself pleads with God for us, in groans
                that words cannot express. And God, who sees into our hearts knows what the
                thought of the Spirit is, because the spirit pleads with God on behalf of his people
                and in accordance with his will.

 What we lost through Adam (that intimate close relationship), we have regained through Christ.
If he had chosen not to come to earth this would not have happened.

This grace is unwarranted, unmeritted - one that can't be earned. No matter how good we live our lives, or how much money we give away, if we haven't accepted Christ into our lives we are missing out. This grace is only received when we accept that Christ died for our sins, apologize to God, choose to change our ways, and ask God to come into our lives and dwell in us.