The causes for the hardening of the heart are:

• Sinning and disobedience to Allah.
• Excessive heedlessness (of the religion).
• Accompanying the heedless and the sinful.

All of this is from the causes of the hardening of the heart.

• Obedience to Allah the Mighty and Magnificent.
• Accompanying the righteous.
• Spending one's time with Dhikr, reading the Quran and seeking forgiveness.

Whoever spends his time by (verbally) remembering Allah, reading the Quran, accompanying the righteous and distancing oneself from the heedless and the sinful, his heart would be in a good state and soften, Allah the Elevated said:

{It is with the remembrance of Allah that hearts find tranquility