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 dont 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.