Why Jesus ?
God created us for His pleasure. He desires sons and daughters who will bring Him much glory, and experience a loving relationship with Him. God is absolutely and perfectly righteous and He requires righteousness of us as well. Righteousness is the fulfilling of God's moral law.
We have all sinned and are sinful by nature, and because God is just, He must judge us according to His law. The consequence of sinning against an infinite God, is infinite punishment. This means eternal torment with no hope and there is nothing we can do by ourselves to make us right with God.
However, there is great hope! God sent His son, Jesus Christ, to save people. He lived a perfect life, achieving righteousness, died a sacrificial death and was raised to life by His Father.
Christianity is all about an exchange. For everyone who turns from their sin and believes in Jesus Christ, God places their sin on Christ and He is punished on their behalf. His righteousness is given to them as a gift. They are reconciled to God and receive eternal life.
'God made him who had no sin to be sin for us, so that in him we might become the righteousness of God.'
2 Corinthians 5:21 New International Version