Well, one reason is because we are taught by well-meaning pastors and teachers that we are still sinners (although saved by grace) and that sinners sin. Actually this is not true. Being a sinner is a spiritual condition, not the result of a specific behavior. If you are born again In Christ, then your spiritual condition is one of a new creation and not a sinner. Yes, you were once a sinner and you have been saved by grace, but you are now new! This is your new and current/eternal spiritual condition - remember that! To believe anything less than this leaves us with a guilt-ridden conscience causing us to wonder if we truly are forgiven and whether Jesus' work on the cross really is finished. Well, of course it is, because He has said so!
Now to the issue of sinful behavior: basically new creations in Christ still sin. God did not remove the sin nature in our flesh, instead, He condemned it in the body of Jesus. If the sin nature had been removed, there would be no reason to walk in faith, and faith is what pleases God. Hebrews 11:6.
So what does this mean that sin in the flesh was condemned by God? It means that sin in the flesh has been deprived of its power to condemn us and make us feel guilty. God condemned sin in the flesh, thereby depriving it of its power to keep us in bondage to guilt. When we do sin and that sin in the flesh attempts to accuse us, we have an advocate (Jesus) with the Father who intercedes for us and reminds us through the Holy Spirit that we are forgiven! We are convicted that we are righteous in Christ and no longer condemned. We are urged to hold on to a clear, clean conscience because of the work done on the cross.
This is a novel idea - to walk around this earth constantly knowing we are forgiven and maintaining a clear, clean conscience before God and man! That is a miracle from Jesus! Can you imagine how wonderful our lives would be if we manifested this high and lofty revelation every day of our lives? The church would be much more powerful and would operate in a glorious, supernatural state that the world needs to see. Paul says in Romans that all of creation groans for the manifestation of the sons of God! Romans 8:19.
So the next time you slip up and fall into sin, keep your eyes on Jesus and remember that the sin in your flesh has been condemned in the body of Jesus and now has no more power over you! Mediatate on that truth until it becomes a reality in your life !