Christ told them that the Law was added because they had hardened their hearts toward God. I have not completed my study of this but the indications are that stubborn and hardened are closely related.
The implications are vast here. They seem to be falling under the mercy and grace of God. We as a people are not as responsive to Him as we should be – I am speaking for myself now. As stubborn hardened people we tend to do our own thing. God seems to be giving us some room to be stubborn while He continues to pursue us.
While that seems really gracious of Him, it did not work out so well for Pharaoh when he hardened his heart against God in Exodus 8:15 (yes I know that both God and Pharaoh are credited with hardening Pharaoh’s heart).
As a people we are always looking for loopholes in the Bible. If we do not find them, we make them. That was what Matthew 5 – 7 was all about, Christ closing the loopholes the Pharisees had created in the Law. It seems to me that the better course of action is to soften my heart toward God. The only way I know to do that is to prayerfully spend time in His Word.