The most common reason for disbelieve is arrogance. Read these verses:
16:22 Your Allah is one Allah. as to those who believe not in the Hereafter, their hearts refuse to know, and they are arrogant. إِلَهُكُمْ إِلَهٌ وَاحِدٌ فَالَّذِينَ لاَ يُؤْمِنُونَ بِالآخِرَةِ قُلُوبُهُم مُّنكِرَةٌ وَهُم مُّسْتَكْبِرُونَ
16:23 Undoubtedly Allah doth know what they conceal, and what they reveal: verily He loveth not the arrogant.
لاَ جَرَمَ أَنَّ اللّهَ يَعْلَمُ مَا يُسِرُّونَ وَمَا يُعْلِنُونَ إِنَّهُ لاَ يُحِبُّ الْمُسْتَكْبِرِينَ
16:24 When it is said to them, "What is it that your Lord has revealed?" they say, "Tales of the ancients!"
وَإِذَا قِيلَ لَهُم مَّاذَا أَنزَلَ رَبُّكُمْ قَالُواْ أَسَاطِيرُ الأَوَّلِينَ
16:25 Let them bear, on the Day of Judgment, their own burdens in full, and also (something) of the burdens of those without knowledge, whom they misled. Alas, how grievous the burdens they will bear!
لِيَحْمِلُواْ أَوْزَارَهُمْ كَامِلَةً يَوْمَ الْقِيَامَةِ وَمِنْ أَوْزَارِ الَّذِينَ يُضِلُّونَهُم بِغَيْرِ عِلْمٍ أَلاَ سَاء مَا يَزِرُونَ
Arrogance can be the only cause or magnify other causes. These all reasons of Disbelieve:1- Plan arrogance: like Pharaoh he believed he was God without any evidence.2- No enough evidences: there is arrogance as well since evidences are abundant see in my blog: The case for God is compelling.3- I am a good man and if there is God I ought to go to heaven. This is also arrogance if your are not looking for God why do you think he should care about you.4- I hate Muslims, Arabs,...... I do the same hate some of them. But I hate some Americans and I love many. Arrogance is to dismiss the most spreading religion that could grantee your eternity for hating some people.5- I do not like the restrictions of religions. Many Muslim sinners do the same. It is better for God to be a sinner than to deny him.6- I think if there is a just God there will be one religion to all people in all time. All prophets of the Bible and Quran have same message of one God and good and evil. It is up to God who gets his messages and at what time. There were always warns of evil and good. Even Egyptians believed in the second life and made mummies and pyramids to glorify the dead. If you examine the very fine details of creation you should be assured that God will be able to judge everyone of us according to all fine details of his culture and religion.7- I am not to bother or waste my time for religion. You are very wise if you are right if you are wrong you are in bad shape since arrogance is God first enemy. It is Satan first sin and it is the sin Satan want us to have to loose our eternity.
8- Christianity says no one can come except through Jesus. That is right, Muslims are extension to Jesus and praying for his second coming. Quran and Bible are from the same God of Israel.
9- Too much war in relgions and I do not like to be a part with. You do not need to fight anyone if you keep faith in God and isolate yourself from wars you are saved. Most wars for other reasons mostly political.