Both of these parties wanted to be in control. They thought they should be the head and didn't like each other.
Lay Investiture is too broad a topic to cover in a quick answer site like WikiAnswers, below are links to articles about it in WikiPedia and the Catholic Encyclopedia.
The appointment of church officials by kings and nobles rather than by the pope
The appointment of church officials by kings and nobles rather than by church officials apex
Emperors and kings disagreed with the Popes over who should appoint bishops. The disagreement lead to the Investiture controversy.
THey gained more power and got to have some control of the church. I am working on the same one
Rivalry between french and english kings.
Rivalry between french and english kings.
Pope Gregory VII issued the Dictatus Papae in 1075, which outlined the powers of the pope and asserted papal supremacy over secular rulers. He forbade kings from exercising authority over church appointments and insisted that only the pope had the power to appoint bishops and invest them with spiritual authority. This conflict with secular rulers, particularly with Emperor Henry IV, contributed to the Investiture Controversy, a significant power struggle between church and state during the Middle Ages.
Through lay investiture, kings gained significant control over the appointment of bishops and abbots, effectively increasing their influence over the Church and its resources. This practice allowed monarchs to ensure loyalty from church leaders, who often held substantial power and land, thereby reinforcing the king's authority and stabilizing his rule. Additionally, by appointing clergy, kings could use religious institutions to support their political agendas and consolidate their power within the realm.
cause they are kings?
Henry wanted the right to name new bishops in his kingdom. Gregory stated that only the pope could elevate bishops. It became known as the Lay Investiture Controversy.
The Concordat of Worms was an agreement between the Church and the Holy Roman Empire, signed in the City of Worms, in Germany, in 1122. It brought an end to the Investiture Controversy and recognized the right of the Church to appoint its own bishops. It was an important step toward the idea of separation of Church and State, and was an blow to the belief in the divine right of kings.