Commentators explain that a 'hard heart' is not just stubbornness, but an active refusal to listen to God's voice. It's a state where truth, preaching, and even life's trials no longer make a spiritual impression. Scholars like Albert Barnes and John Gill note this hardness is often self-inflicted through making excuses, deliberately sinning to quiet one's conscience, and directly resisting the Holy Spirit. It is the root of disobedience.