In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Imaam Ibnul Qayyim [rahimahullaah] said:
A slave is not afflicted with a punishment greater than the hardness of the heart and being distanced from Allaah. The fire of (hell) was created to soften the hardened hearts. The farthest hearts from Allaah are the hardened hearts. When the heart is hardened, the eyes become lacking in (discernment, perception, crying out of fear of Allaah etc). The hardness of the heart occurs due to four [affairs] when their limit is exceeded: Food, sleep, speech and mixing [with the people]. And just as the diseased body is not benefited by food and drink, likewise the heart is not benefited by admonitions when it is afflicted with [evil] desires. Whoever desires purification of his heart then let him give precedence to Allaah over his desires. The heart that is attached to desires is veiled from Allaah depending on the strength of its attachment to [such desires]. [Ref 1]
Imaam Ibnul Qayyim [rahimahullaah] also said:
The heart does get sick just as the body gets sick and its cure is found in repentance and bring keen [to obey Allaah and the Messenger etc]. The heart does become rusty just as a mirror becomes rusty, and remembrance of Allaah polishes it. The heart does become unadorned just as the body, and fear of Allaah beautifies it. The heart does suffer from discomfort just as the body suffers from discomfort due to hunger, and its food and drink are knowledge and awareness of Allaah, love of Allaah, reliance upon Allaah, turning sincerely to Allaah in repentance and obedience with Tawheed, and carrying out acts of obedience to Allaah. [Ref 2]
Imaam Ibn Qudaamah [rahimahullaah] said:
Know that every limb was created to perform a specific action, so a sign of its [i.e. any limb] sickness is when guilt is established due to performing [an evil] act. A sign of the hand’s sickness is when it is guilty of striking unlawfully; a sign of eye’s sickness is when it is guilty of staring unlawfully; a sign of the heart’s sickness is when it is guilty of not doing what it was created for, such as acquiring knowledge and wisdom; love of Allaah, worshiping Allaah alone and preferring that over every other desire. Therefore, if a person was to possess knowledge of everything but he does not know Allaah (Gloried be He), then it is as if he knew nothing. The sign of knowing Allaah is to love Allaah, for whoever truly knows Allaah will love Allaah, and a sign of having love for Allaah is that one does not give precedence to anything over Allaah. So whoever prefers something from the beloved things over Allaah, then his heart is sick, just like the stomach that prefers mud and not bread, due to losing the desire for bread. [Ref 3]
[Ref 1: Source: (Al-Fawaa-id: page: 153-154). slightly paraphrased]; [Ref 2:An Excerpt from ‘Al-Fawaa’id 143’]; [Ref 3: Source: An Excerpt from Mukhtasar Minhaaj Al-Qaasideen’ page. 64. Slightly paraphrased]