When we do not get what we want, we think that God does not care for us. We want God to do our bidding and pander to our whims and fancies. So when we have to go through suffering, we blame God. And if God does not restore our fortunes, we turn away from Him. Why do we lose faith in God? It is forgetfulness. The second reason why people harden their hearts is because of sin. When sin accumulates, we harden our hearts to goodness and truth. Thirdly, people harden their hearts because of pride. Intellectualism is the greatest pride of humanity today. We think we have the answer to everything and everyone presumes he is right. When we are arrogant and think so highly of ourselves, we harden our hearts to truth and goodness.