Any new movement or cause that emerges for the first time in a society will be considered strange and unfamiliar and will be met with skepticism, apprehensiveness, and doubt and in most cases will be met with opposition, rejection and disapproval. The wider the gap between the society and its beliefs, customs and laws and the ideals of the new movement, the greater the conflict and aversion between the two.
If we imagine the pre-Islamic nation of Arabs that the Prophet Muhammad (Peace be upon him) was in the midst of, full of corruption, deviant beliefs, and foolish customs, we understand that the people were engrossed in idolatry. We can then we compare that with the message that the Prophet Muhammad (Peace be upon him) was sent with by his Lord; a message that was complete, beautiful, pure and purifying; a message that called on the people to reject all of their false deities and to make all of their worship for Allah alone; a message that called on the people to stop worshipping rocks and trees and idols that don't see or hear or benefit those who worship them. A message that called on the people to leave all of their deviant beliefs related to angels and jinn that were deeply rooted in pre-Islamic Arab culture; a message that called on the people to change the laws and the common customs of the time.
If we conceptualize all of that and the fact that Islam sought to completely change the society and return the people to correct beliefs and practices, then we understand that conflict was unavoidable. The enormous difference between pre-Islamic society and Islam was enough as a cause for the difficulties that Islam and the early Muslims faced. However, there were some other distinct and specific causes that deserve mentioning because of their importance in explaining the hardships early Muslims had to endure.