My first reason is that the Catholic Church was taking advance of the people and was cheating people. Catholics believed that Priests could forgive people’s sins in exchange for a gift called “indulgences”. These gifts or indulgences could be money, clothes, decoration to the church or even to repair the church. Some people even gave valuable and precious things such as chalice to their priest in exchange for their sins to be forgiven. However, I think the Catholics were cheating these people as only God can forgive your sins and the Priests were not actually forgiving their sins. They were the ones who were benefiting from this and the people were suffering from this.
Also, Catholics believed in Purgatory. They believed that when you died you went to Purgatory which is basically believed to be a waiting room. It is there where you were decided to either go to Heaven or Hell. This made the Catholics even more worried and convinced them that they should get their sins forgiven more often which meant again the Catholic Church would benefit and receive gifts.
I also agree with Martin Luther as the Priests of the Catholic Church lived in luxury and were thought to be more important and more superior to anyone else. This is ludicrous as Priests are not special. They take services and make it easier for the people to understand about God. They should not live in luxury or to be superior and they should live normally but to be appreciated.
To conclude I think the Catholic Church cheated the people who worshipped God and Martin Luther was right to protest against it.