Martin Luther came to feel that this was the wrong way of seeing things. It was selfish. He thought it was wrong for a person to behave properly for the sole purpose of securing a place in Heaven. To put it in the author’s words, "it is God who saves us, not we who save ourselves."
Luther felt that it should be the other way around. God had already granted us salvation. As long as people have faith in this salvation, they'll be okay. Luther believed that if people truly believed in this salvation, they would willingly live a morally good life. They would accept this gracious gift from God, and try to do right by God purely out of goodwill. It is in the true believer's nature.
As one can imagine, this kind of thinking would raise serious eyebrows when it came to church indulgences. If a person was already saved, why would he need to pay the church more money to be more saved?
(Full disclaimer: The Reformation is a very deep subject. This was just one thing that stuck with me and marked a key theological difference between the Catholic Church and Martin Luther's worldview.)
Sources:
"The Reformation" - Patrick Collinson
