Why did monarchs and popes struggle with each other for control of society?

For the monarchs, it was generally about economics. For the papacy, it was generally about religion. The monarchs believed they had the right to tax church priests and other important Catholics, something that the popes didn't like at all. The popes believed that God had set the pope above ALL rulers, and therefore had more authority than all the kings, something the monarchs didn't like. During this time, there were also plenty of schisms going on that weakened the church, so the popes became more and more desperate to gain more power in the secularizing age. Overall, the conflict was over who had more power.