Salam Alaykum,
I think it is a mixture of matters. A few would be:
1) Incorrect information on Islam. Like visiting a anti-muslim website and been fed mis-information, that causes them to dislike Islam.
2) Absence of will or determination to follow the dean, and instead follow the evil desires, that are repeatedly amplified by the WasWasa of the damned Shayateen (devils). Everytime a desire is followed that is forbidden in Islam, the Iman decreases until there is non left.
3) And one main reason, ARROGANCE. The most evil kind of arrogance is rejection of the truth and not following it. In this context, many verses were revealed to scold arrogance and the arrogant people. Examples are: On the Day of Judgment, it is said to the arrogant people in surat Ghafer, (Verse 76), what can be translated as,
"Enter the gates of Hell to abide therein, and (indeed) what an evil abode of the arrogant!"
"And turn not your face away from men with pride, nor walk in insolence through the earth. Verily, Allah likes not each arrogant boaster. (31:18)
“And the slaves of God are those who walk on the earth in humility and calmness, and when the foolish address them (with bad words) they reply back with mild words of gentleness.†(Quran 25:63)
Sallam