Why did lay investiture cause a struggle between kings and popes?

The main cause of the struggle between kings and popes during the lay investiture was that it caused a stir because of the question of who should have the most power in a time where the Church was very highly regarded and the monarchy was as well and who should pick bishops and abbots. The popes believed it was their right to do so, but the kings believed otherwise. It was then decided that the pope could carry this out, although the Emperor retained significant power over the Church.