In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Imaam Ibnul Qayyim [rahimahullaah] said:
 Ignorant of What The Truth Is: This reason is mostly what overwhelms the majority of people, for indeed the one who is ignorant of something will show enmity to it and its people. If in addition to this reason, he [i.e. the person] hates the one who commands him to [follow] the truth, harbours enmity and envy, then this [i.e. the reason behind being prevented from accepting the truth] becomes more intense. If in addition to this, it [i.e. the truth] opposes the ones close to his heart, his customs, the one who nurtured him, the ways of his forefathers and the ones he loves and exalts, then this [i.e. the reason behind being prevented from accepting the truth] becomes even more intense. If in addition to this, he carries a wrong perception that the truth he is being called to [is an obstacle to] his status, honour, desires and goals, the [reason behind being prevented] from accepting the truth becomes very intense. If in addition to this, he fears his companions, his family and his people [i.e. due to the harm he thinks will come from them] against him, his wealth and status – just as what happened to Heraclius, the king of the Christians in Shaam during the era of Allaah’s Messenger [sallal-laahu-alayhi-wasallam], then the reason [behind being prevented from accepting the truth] increases even greater [than before]; for indeed Heraclius knew the truth and he had a desire to accept Islaam, but his people did not obey him; so he feared for himself and thus chose disbelief over Islaam after guidance was made clear to him.
Envy: Envy is one of the greatest causes [behind being prevented from accepting the truth], for indeed it is an ingrained disease in the soul. The envier sees that the one he envies has been given virtue over him or has been given the likes of that which he has not been given; therefore this envy prevents him from complying with [the truth]. Iblees was not prevented from prostrating to Adam [i.e. out of respect, when Allaah commanded him and the angels to do so], except out of envy; for indeed when he saw that Aadam was given virtue over him and was raised above him, he chose disbelief over Eemaan, even though he used to be in the company of the angels.
This disease [i.e. envy] is what prevented the Yahood from believing in Eesaa the son of Maryam [alayhimas-salam], even though they knew -without doubt – that he is a Messenger of Allaah, who came with clear proofs and guidance; but envy led them to choose disbelief over Eemaan and complied with it, even though they were a nation amongst whom were Rabbis, scholars, Zuhhaad [i.e. those who abstained from those lawful but unnecessary pleasures of the worldly life], judges, kings and rulers. This [was their behaviour] when the Messiah came with the legislation of the Tawraat and he did not bring a legislation in opposition to it, nor did he fight them; rather he came to make lawful some of that which was made unlawful for them in order to lighten the burden [upon them]- out of mercy and kindness. He came to complete the legislation in the Tawraat, but they all chose disbelief over Eemaan.
Fear of losing one’s status and prestige:
Conversation between Abu Jahl and His Nephew Al-Musawwar Bin Makhzamah
Al-Musawwar Bin Makhzamah: ‘’O my uncle! Did you use to accuse Muhammad of lying before he started saying what he says now [i.e. his call to Tawheed]? Abu Jahl: “By Allaah, indeed Muhammad was a trustworthy young man amongst us and we have never experienced lying from him. Al-Musawwar Bin Makhzamah: “O my uncle! So why is it that you do not follow him?” Abu Jahl: “O son of my sister! We competed with Banu Haashim in nobility [i.e. which clan is more noble], so they used to feed [the pilgrims] and we used to feed the [pilgrims]; they used to provide water for [the pilgrims] and we used to provide water for [the pilgrims]; they used to give protection [to the destitute] and we used to give protection [to the destitute] until we were equal [i.e. in competing one another for nobility]. Then they said, “There is a Prophet from us”, so when are we going to get something similar to this!!
Conversation between Abu Jahl and Akhnas Bin Shuraiq
Akhnas Bin Shuraiq said to Abu Jahl on the day of Badr [i.e. the battle of Badr]: “O Abul Hakam! Inform me about Muhammad, as to whether he is a truthful person or a liar, for indeed there is no one here from Quraish who will hear our speech? Abu Jahl replied to Akhnas: “Woe to you! By Allaah, indeed Muhammad is a truthful person and Muhammad has never lied, but if Banu Qusay take the leadership, the position of gatekeepers of the Kabah, providers of water for the pilgrims and Prophet hood [i.e. the Prophet Muhammad is from their clan], then what will the rest of Quraish have?! [Source: An Excerpt from Hidaayatul Hayaaraa Fee Ajwibatil Yahood Wan-Nasaaraa’ pages 17-19. slightly paraphrased]
Al-Allaamah Abdur Rahmaan Al-Mu’allimee [rahimahullaah] said:
The Religion comprises of various levels- refraining from prohibitions and fulfilling what [one] has been commanded; acknowledge truth, belief in it and acquaint [oneself] with it. To oppose [one’s] desires for the sake of the truth is something clear, for indeed most of what is prohibited are the lusts and [evil] pleasures. And it may be that a person does not really desire the pleasure found in those [prohibited] things, but he merely desires them due to an opposition [he faces]. To oppose [one’s] desires – by way of action – for the sake of the truth is a manifest affair due to the obligation and difficulty it involves. This can be viewed from different angles, as follows:
Firstly: A person sees that by acknowledging the truth, it would necessitate that he has acknowledged that he was upon falsehood, because a person is nurtured upon a religion, creed, school of thought or views, which he acquired from his nurturer and teacher, considers it to be the truth and followed it for a long time. Then when it becomes clear to him that [that religion, creed, view, or school of thought] is false, it becomes difficult for him to acknowledge it, just like when his forefathers, ancestors or the one he follows are upon a [particular] way and then its falsity is clarified for him- because he sees their deficiencies necessitate his own deficiencies, and an acknowledgement of their misguidance or errors necessitates his own deficiencies.
Secondly: It may be that clinging to falsehood gave him prestige, fame and a livelihood; so it becomes difficult for him to acknowledge that [what he is upon is] falsehood and thus all those benefits [i.e. the fame, prestige, livelihood etc] would disappear.
Thirdly: Pride: It may be that a person is upon ignorance or falsehood, then another person comes along and clarifies the proofs for him; so he sees that by acknowledging such proofs, it would necessitate that he is lacking [in understanding] and that it was that person who guided him. And due to this we find that it is not difficult for some of those attributed to knowledge to acknowledge their mistakes when it becomes manifests to them in their researches and studies, but it becomes difficult for them [to acknowledge] if it was others who clarified such mistakes for them.
Fourthly: Envy: [This may occur] when another person clarifies the truth for him, so he sees that by acknowledging such truth it would necessitate an acknowledgement of the clarifier’s virtue, knowledge and correctness due to that clarification, and thus that becomes [something] great in the eyes of the people and many people [begin] to follow him [i.e. the person who clarified the truth]. So you will find some of those attributed to knowledge being eager to prove the mistakes of other scholars even if that is done by way of falsehood, due to envy and seeking to diminish their status amongst the people.
To oppose [one’s] desires for the sake of the truth -in affairs of knowledge and creed- can indeed be difficult to accomplish, therefore it requires [sincere] research and contemplation; and –with regards to that- one is in need of asking the scholars and benefiting from them; adhering to Taqwah, seeking the Tawfeeq of Allaah and guidance. [An excerpt from ‘At-Tankeel Bimaa Fee Ta’need Al-Kawthariy Minal Baatil’ 2/180-181 with the checking of Imaam Albaanee (rahimahullaah). Slightly paraphrased and abridged]