In any other profession, you can be too merciful. Imagine if you worked in a bank but let everyone off from paying their loans. Imagine a convenience store clerk who let anyone with a sob story take a free loaf of bread. However, in the Sacrament of Confession, we priests can never be too merciful because we are simply instruments of Christ who is all-merciful. We priests have the opportunity to forgive everything no matter how minor or grave.
I think I’d let too many people off the hook if I were in business.