There are at least two ways to look at this question.
On the one hand, we can look at the immediate causes of the Reformation. The Reformation was caused by the unhappiness that Martin Luther felt towards the Church, particularly because of the sale of indulgences to raise funds to build St. Peter's Basilica. The Reformation was further caused by the fact that the Church chose to come down hard on Luther for his ideas, thus driving him to push for the creation of a new faith.
To take a broader view, we can say that the Reformation was caused by the Church's inability to provide what its members needed. The Church had become fairly corrupt by the 1500s and often seemed to be more concerned with temporal power and wealth than with the spiritual needs of its members. This failure on the Church's part, when combined with the new attitude of the Renaissance, which promoted the questioning of authority, can be seen as the more ultimate cause of the Reformation.