The destruction of the heart occurs by feeling love and security in this Dunya and neglecting the aakhirah.
The strengthening of heart occurs by fear and reflecting about Allah SWT and aakhirah.
If the heart gives up the pleasures of the Dunya, it starts upon chasing the pleasures of the Hereafter. Such a person becomes an obedient servant of Allah.
If the heart becomes content with the pleasures of the Dunya, the longing for the pleasures of the hereafter stop.
PASSIONATE LOVE
Passionate love is a psychological sickness, and when it's effects become noticeable on the body, it becomes a sickness that afflicts the mind also. Either by afflicting the mind by the likeness of agony, or afflicting the body through weakness and emancipation. But the purpose here is to discuss its effect on the heart.
Passionate love is the fundament that makes the soul go after that which would harm it.
If he is not satisfied by that then he is grieved, and if he is satisfied then his sickness increases.
Therefore the only cure for the sick lies in his removing the sickness by removing this blameworthy love from his heart.
WHAT HAPPENS WHEN A HEART BREAKS
The heart afflicted with this love is harmed by its connection to the loved, either by seeing, hearing, touching or even thinking about them. And if he were to hold back the love then the heart is hurt and grieved by this, and if he gives in to the desire then the sickness becomes stronger and becomes a means through which the grievance is increased.
This is in contrast to the one whose objective is not met, and the heart breaks, for this failure results in removing the satisfaction that would have strengthened the sickness. Thereby the desire for the loved one is weakened, and the love for them is weakened. This is because the person definitely intends that there be action accompanying his desire, otherwise all his desire would become as minute as just whisperings of the soul, unless the person speaks or looks or is in contact.
PARADISE IS THE ABODE FOR THE ONE WHO HOLDS BACK SUCH LOVE
As for the one who is afflicted with this passionate love but holds back and is patient, then indeed Allah will reward him for his taqwa as occurs in the hadeeth:
"The one who passionately loves someone yet holds back, conceals this and is patient, then dies upon this, will be a martyr."
(A da'eef hadeeth . refer to the discussion concerning its authenticity in al-Jawaab al-Kaafee and Rawdha al-Muhibbeen of Ibn al Qayyim and Silsilah ad-Da'eefah of al-Albaanee.)
Although the above hadith is weak but it is known from the evidences of the Shareeah that if one were to hold back from performing that which is unlawful, be it looking, speaking or acting, and he conceals this and does not articulate it so as not to fall into that which is prohibited and he is patient in his obedience to Allah and keeps away from disobedience to Allah, despite the pain that his heart feels due to this passionate love (similar to the case of the one who is patient through a calamity), then indeed this person would gain the same reward as those who have feared Allah and been patient.
"Verily, he who fears Allah and is patient, then surely Allah makes not the reward of the doers good to be lost."
[Surah Yusuf: Ayah 90]
"But as for him who feared the standing before his Lord, and restrained himself from impure evil desires and lusts. Verily, Paradise will be his abode."
[Surah an-Naaziaat: Ayah 40]
SUCH LOVE CAN BE SINFUL
When the soul loves something, it will do all it can to attain it, so the one who does this out of having a blameworthy love then this action of his would be sinful.
As one poet affected by this said:
For the sake of a Sudanese girl he loved Sudan to the point that he loved the black dogs due to his love of her.
Ibn al Qayyim al Jawziyyah's al Fawaaid (p111-112)
SUCH LOVE TAKES YOU AWAY FROM ALLAH
The slave is not afflicted with a punishment greater in severity then the hardening of the heart and being distant from Allah. For the Fire was created to melt the hardened heart. The most distant heart from Allah is the heart which is hardened. If the heart becomes hardened, the eye becomes dry.
If four matters are exceeded in quantity, beyond what is necessary, the heart shall become hardened:
Food, sleep, speech and desire. A body afflicted by disease does not derive nourishment from food or water, similarly a heart diseased by desire does not benefit from admonishment and exhortation.
Whosoever desires to purify his heart, then let him prefer Allah instead of his desires.
SOLUTION TO CURE SUCH A HEART
They (the disobedient ones) preoccupied their hearts in the pursuit of the Dunya, would that they preoccupied them with Allah and the hereafter, then surely they would have opened the Qur'an and reflected upon the meaning of His glorious verses. If they did this, their hearts would have returned to their Master with wisdom, marvelously curious and with deep thoughtfulness about the life in the aakhirah.
If the heart is nourished with remembrance of Allah, its thirst is quenched with contemplation (deep thinking) and cleansed from corruption, it shall witness remarkable and wondrous matters, inspiring wisdom.
Yearning for Allah and His meeting is like the gentle breeze blowing upon the heart, extinguishing the burning heart. Whosoever caused his heart to settle with his Lord shall be in a calm and peaceful state, and whosoever set his heart on people of the Dunya, shall be disturbed and excessively nervous.
Because the love of Allah shall not enter a heart, which contains the love of this world, just as a mountain cannot pass through the eye of a needle.
Hence, the most beloved servant before Allah is the one whom Allah chooses as His servant, in whose heart Allah puts His love, whom Allah makes purify his worship for Him, who dedicates his objectives for Allah, whose tongue for Allah's remembrance, and his limbs for Allah's service.
The heart becomes sick, as the body becomes sick, and its remedy is al-Tawbah and protection [from disobedience].
It becomes rusty as a mirror becomes rusty, and its clarity is obtained by deeply thinking about the paradise and hell. It becomes hungry and thirsty as the body becomes hungry and thirst, and its food and drink are knowledge, love, dependence, repentance and servitude.
-Slightly Abridged Excerpt from 'Diseases of the Heart and its cures' by Sheikh Ibn Taymiyyah
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