There are two main reasons why Henry VIII broke with the Catholic Church and started the English Reformation. Neither of them really has to do with theology as Henry had previously won the pope’s adulation as a “defender of the faith.”
First, Henry broke with the Catholic Church over his desire for a divorce. He and Catherine of Aragon had been unable to produce a male heir. Therefore, he wanted to divorce her and take a different wife who, he thought, would give him a son. The Church did not want to grant him the divorce and so he broke from it.
Second, Henry wanted to have more control over the church in his kingdom. Monarchs in these days were constantly wanting to have such control both because they church was rich and because it was very important to the lives of many people. Monarchs did not like having an independent power in their kingdoms. Henry’s desire to exert more control over the church also helped lead to his break with the Catholic Church and his creation of the Church of England with the monarch as its head.