What is the reason for the heart's fluctuation between tenderness and hardness despite adherence to acts of worship and avoidance of sins?

To cure hardness of heart, one must remember death and its terrors, and Paradise and its delights, and Hellfire and its torment. It is also recommended to ponder Allah's beautiful names and attributes, and to recite the Quran with humility and contemplation, and to associate with righteous people. Most importantly, one should make abundant supplication and ask Allah Almighty for guidance and for the rectification of the heart.