Because it's important. Not just important, but very important; of first importance!
It's important because Jesus took MY punishment for ME. He took my sin, and gave me His righteousness. He took the Wrath of God that was directed at me, and presents me spotless before the throne.
That's what Substitutionary Atonement is all about.
When I consider my life - my thoughts, words and deeds- and compare it to what God requires -pure, spotless righteousness- I know that I am lost and undone. I cannot stand before a Holy God. I cannot even approach His mountain. Not on my own. Not with my own righteousnesses, which are filthy rags in God's sight.
Yet because Jesus was willing to take my sins upon Him, and take the punishment I deserved, and exchange my filthiness for His spotlessness, I can not only know God, but love Him as my Father. I can serve Him in this life, knowing that Jesus is my mediator, Who stands before God, reminding Him that He already paid for that gossip, that snide remark, that pang of jealousy that plagues my heart.
So, I rejoice in the cross. I delight in the cross. I exult in the Cross of Christ.