It is due to turning away and rebellion.
Sincere repentance (tawba-i nasūḥ) softens the nafs (the lower self) and leads it away from its filthy nature towards gentleness. For the nafs becomes clarified through muḥāsaba (self-accounting) and murāqaba (spiritual vigilance), and its burning fire is extinguished by following caprice (hawā). Again, the nafs, through the state of itmi'nān (serenity), attains the locus and station (maqām) of riḍā (contentment with God's decree), and becomes tranquil in the face of the flow of destiny (qadar).
Sahl al-Tustarī said: Sabr (patient endurance) is to await the opening of a way out by Allah, and this is the most virtuous and exalted of services.
Know that when the Exalted Truth (Ḥaqq Taʿālā) wishes to strengthen a servant by granting him His wisdoms, He bestows upon that servant the lights of His attributes and Himself through ikhsān (spiritual excellence), and thus decrees descend. At that time, the lights go before that servant, and he is with his Lord, not with his nafs. Thus, the servant becomes strong against his hardships and resilient against the desires of the nafs. For Allah assists them in submitting to destiny and in continually relying upon the flow of light. Whether you say that Allah aided them in bearing the rulings that open the door of exposition (bayān), or that He aided them in bearing the tribulations that are the arrivals (wārid) of ikhsān, it is the same.
Whether you say that He gave them strength in accepting destiny as a witness (shāhid) to their good choices in tawakkul (reliance upon God), or that He strengthened them with sabr in the state (ḥāl) of knowing the existence of His knowledge in the presence of His decree, or that He made them patient by the presence of His Beauty (jamāl) in the manifestation of His acts, it is all valid. Their sabr towards destiny is due to their knowledge that sabr leads to riḍā, or you may say that their sabr towards destinies is the unveiling of veils and obstacles.
If you wish, you may say that their only strength against the difficulties of obligation (taklīf) is the arrival of the secrets of divine disposal (taṣrīf), or that their sabr towards their destinies is because they know that in those destinies are Allah's grace and ikhsān. These ten reasons necessitate that the servant be patient, show steadfastness (thabāt) towards his Lord's rulings, and be strong at the time of their arrival. He is the One who grants all these as His own ikhsān. He is the One who bestows this upon the people of divine favor (ʿināya). (Tanwīr, 4)
Sabr leads to riḍā; thus, whoever is patient in following Allah's rulings, Allah grants him riḍā, and he endures the hardships of (destiny's) rulings for the sake of Allah's riḍā, just as one drinks a bitter medicine to obtain healing in the end. (Tanwīr, 6)
The stations (maqāmāt) of yaqīn (certainty) are nine: they are tawba (repentance), zuhd (asceticism), sabr, shukr (gratitude), khawf (fear), riḍā, rajāʾ (hope), tawakkul, and maḥabba (love). It is not correct to consider any of these stations as being nullified by planning or preference with Allah. (Tanwīr, 8)
Both sabr and shukr are only valid by abandoning planning. For the one who is patient is patient against what Allah does not love. What Allah does not love is to plan and prefer against Him. Sabr is of various kinds: sabr against prohibitions, sabr in fulfilling commands, sabr against planning and preference, and so on.
Alternatively, you may say: sabr against human pleasures, sabr in fulfilling the requirements of servitude, and sabr in fulfilling the requirements of servitude by abandoning planning against Allah's decree.
Similarly, shukr is only valid for a servant who abandons planning against Allah's decree. As Junayd said: Shukr is not rebelling against Allah despite all His blessings. If Allah had not distinguished you from others with intellect and made it a means for your perfection, you could not have planned against Allah's decree. For inanimate beings and animals, since they have no intellect to foresee their end and pay attention to it, they have no planning against Allah's decree. (Tanwīr, 9)
Allah says: "Seek help through sabr and prayer. Indeed, prayer is difficult except for the humble." (al-Baqara 2:45). Because sabr is needed in prayer, sabr and prayer are mentioned together. Sabr is needed to perform prayers at their proper time, to fulfill the sunnah and obligatory acts properly, and to guard the qalb (the heart) from heedlessness during prayer. For this reason, Allah said, "Prayer is difficult except for the humble," emphasizing prayer rather than sabr, thereby drawing attention to being patient in prayer. (Tanwīr, 46)
Doing good to the orphan without expecting anything in return and enduring hunger are also righteous deeds. (al-Furqān 25:11)
Sabr is a human characteristic. Since animals are dominated by desires (shahawāt), and are imperfectly created, it is not conceivable for them to be patient. For angels, who are perfect beings, sabr is also not applicable; they only yearn for the Divine Lordship (Rubūbiyya) and are never subject to any carnal deficiency that would require struggle (mujāhada) or keep them from the Divine Majesty (Jalāl). (Qudāma, 285)
Sabr is divided into two: The first is bodily sabr, such as burdening the body with difficult tasks or performing strenuous acts of worship. The second is nafsānī sabr, shown against things the nature enjoys and the caprice (hawā) demands. If sabr is against the desires of the stomach and sexual organs, it is called chastity (ʿiffa); if against fighting, it is courage (shajāʿa); if in restraining anger, it is forbearance (ḥilm); if against a severe calamity, it is breadth of chest (saʿat al-ṣadr); if in concealing something, it is keeping a secret (kitmān al-sirr); if against surplus provision, it is zuhd; if against simple pleasures, it is contentment (qanāʿa). (Qudāma, 286)
The pain of sabr invites relief, while shukr invites joy. These two are opposites. Know that just as blessing exists, so does tribulation. Not every tribulation is commanded to be endured with sabr—for example, kufr (disbelief) is a tribulation, but there is no meaning in being patient with it. The same applies to acts of disobedience (maʿṣiyya). However, the disbeliever does not know that his kufr is a tribulation; he is like a sick person who feels no pain because the illness has overtaken him. The sinner, however, is aware of his disobedience and must abandon his maʿṣiyya.
Man has the power to repel every tribulation. He is not commanded to endure every tribulation with sabr. No one is commanded to refrain from drinking water so that his thirst and pain increase; rather, he is commanded to remove the pain. Sabr is only relevant for the servant in the face of pain that cannot be removed. Thus, sabr in the world is for tribulations that are not absolute. It is possible that something seen as a tribulation may, in another aspect, be a blessing. For this reason, the duties of shukr and sabr may coincide. For example, wealth may be a cause of destruction, as one may be killed for his property. The same applies to health. There is no blessing in the world that cannot, in another aspect, be a tribulation. Sometimes, a servant may be afflicted with a tribulation that contains a blessing. (Qudāma, 310)
Degrees of sabr: The lowest degree of sabr, though difficult, is to abandon complaint. Beyond this is to show riḍā towards hardships, which is the station following sabr. After this comes shukr for tribulations, which is beyond riḍā. (Qudāma, 314)
They have said about sabr: Sabr is to restrain the nafs from tribulation during a test (imtiḥān) and to protect the tongue from complaint by trusting in the beauty of the outcome. It leads to maḥabba (love) and curbs excessive desires, breaking down their barriers. For the elect (khawāṣṣ), it is to take pleasure in the tribulation from the Beloved, and when the secrets of the qalb are unveiled, to taste the pain in the love of the sought thing, to witness the Cause in the causes, to see Allah in the one who inflicts the pain. It is also to attain the lofty manifestation of maḥabba and to be characterized by it in a way that never ends. (Rawḍa, 411)
Sabr is that which absolutely prevents the nafs from complaint. (Ṣufyānī, 334)
Sabr is to remove anxieties and to keep the nafs in the station of ʿubūdiyya (servitude) by holding it firm. (Jāmiʿ, 54)
Sabr is of three kinds: the sabr of the common people is against maʿṣiyya (disobedience), the sabr of the elect (khawāṣṣ) is to persist in obedience, and the sabr of the elect of the elect (akhāṣṣ al-khawāṣṣ) is to be with the Truth (Ḥaqq). (Jāmiʿ, 61)
Know that the roots of the stations of yaqīn attained by the branches of the states of the people of yaqīn are seven: tawba, sabr, shukr, rajāʾ, khawf, zuhd, tawakkul, maḥabba, and riḍā. (Jāmiʿ, 218)
These (transactions that draw near to Allah) are spiritual transactions that arise in the nafs as luminous and firm dispositions after the continuous striving of the qalb. Thus, the nafs begins to attain serenity (itmi'nān) and the qalb to submit willingly (to the Truth).
In the end, the nafs is adorned with the virtues and faculties that are pleasing to the Truth and are the source of good deeds. Among these virtues is sabr towards desired things, which is the virtue of the appetitive faculty. Sabr towards disliked things is also among these virtues, which is the perfection of the irascible faculty.
The initial form of sabr is to keep the nafs steadfast and away from rebellion, and to direct it towards obedience. At the gates, sabr is again to struggle against desires and prevent them, to make bearing acts of worship a habit, and to abandon anxiety in the face of tribulations. In transactions, sabr is to prevent the nafs from becoming accustomed to laziness and to encourage it to submit in guarding the rights of Allah. (Jāmiʿ, 283)
Sabr is not distinguishing between blessing and hardship while the heart is in tranquility. Sabr is to endure tribulations and bear the weight of hardships. Abū'l-Qāsim Junayd said: For a believer, passing from this world to the hereafter is very easy and simple. However, leaving people and being in the presence of the Truth is quite difficult. To walk from the nafs to Allah (after leaving the body) is also difficult. More difficult than all these is to be with Allah. When he was asked what sabr is, he replied: It is to drink a bitter potion without making a face.
It has been said: Sabr is to complain without revealing one's complaint when afflicted by tribulation (i.e., to present one's state only to Allah).
Abū ʿAbdullah Ḥafīf said: Sabr is of three kinds: mutaṣabbir (one who forces himself to be patient), ṣābir (one who is patient), and sabbār (one who is exceedingly patient).
Abū ʿUthmān said: The sabbār is one who trains his nafs to endure what it dislikes. ʿAmr ʿUthmān said: Sabr is to be able to endure being with Allah and to invite and bear every tribulation and hardship that may come from Him.
Ibn ʿAṭāʾ said: Sabr is to maintain good etiquette even amidst calamities. What I have transmitted here is only ten sayings; I have summarized them. (Nabhānī, I, 301)
sadā (ṣadā): Sadā refers to the evils committed due to the constitution (vices) of the nafs, the dark and material forms resulting from its structure, and its self-aggrandizement. (Jāmiʿ)