The word used in the Quran for hardening of the heart is قساوة  (2:74) Qasawat means to be insensitive. The whole matter is about insensitivity. The more sensitive you are, the more you will differentiate between good and bad. This is related to contemplation. People are not sensitive about Paradise because they don’t think much. I haven't found people sensitive about Paradise. But they are sensitive about other things. They don’t read the Quran with understanding. They don’t study Islamic books. People don’t even attend spiritual lectures. If a film actor makes an appearance here, many people will instantly gather. People don’t have the taste to listen to spiritual truths of a higher level. People gather in crowds to listen to third class stuff. We talk about the truth here, but nobody seems interested. This is what produces hardening of the heart. If you don’t think, study or sit with people who tell you about the truth, your sensitivity decreases. It is extremely essential for a person to involve himself in spiritual matters through conversations, study and reflection. If he is in the companionship of the pious, he will be protected from hardening of the heart.