Martin Luther (1483-1546) did not seek to break from the Catholic Church; his concern was to correct what he perceived to be abuses by the church hierarchy, notably the sale of indulgences. However, after his theological studies he became convinced that one's salvation could be by "grace alone," meaning that the church did not have the monopoly of acquiring the blessed state of heaven. This, more than the sale of indulgences, got him into trouble since that argument fundamentally undermined church authority. Refusing to recant his position, he was excommunicated -- he did not break from the church, rather, the church broke from him.
Although others had been critical of the church in times past, the significance of Luther was not simply rooted in his philosophical arguments; rather, by the time he posted his 95 Theses, the printing press had been in use for nearly half a century and literacy was on the rise -- not only did villagers in Wittenberg read his complaints, but also the literate across Europe. Along with the rise of nationalist sentiments across the remains of the Holy Roman Empire and other old political structures, his arguments, taken up by many others dissatisfied with the status quo of church and state began the "Protestant Revolution."
There were a couple of major reasons for Martin Luther becoming disillusioned with the Catholic Church.
One of these was the practice of selling indulgences. The Church would sell people forgiveness for their sins. Luther did not believe this was something that could be sold. He believed that a person could only be forgiven their sins by God's grace.
The second comes out of the first. Luther was concerned about the amount of power given by the Church to its hierarchy. He did not like the way that each level was set above the other (priests above lay people, bishops above priests, etc) and he did not like the way the lower levels were supposed to blindly obey the upper levels.