Paul speaks of the goodness of God with forbearance and longsuffering.  Because we are not reprimanded for sin or immediately punished, people harden themselves to the reality of who and what they are.  The context shows one should be sensitive to the reality of their nature so they do not amass the accumulating stock of divine wrath in the righteous judgment of God. King David and King Nebuchadnezzar are examples of this process.