Because He is a holy God. He expects His children to be holy. But we can't get holy by ourselves. We must have the help of The Father, and The Son, and The Holy Spirit. And they will help us if we want to be helped.
Christians fall everyday.
But you see the Bible is like a mirror, It shows us who we really are. It also shows us how loving, and kind, and merciful our God really is. I can tell you from experience. If you ask the Lord Jesus Christ to come into your heart. You will never regret the decision to except him.