It was the lack of a male heir to ensure the perpetuation of the Tudor monarchy.
Indeed, Henry VIII was married to Catherine of Aragon, who did give him a male heir who unfortunately died within a few weeks of his birth. Their second issue was a female daughter and at the time no English monarch had ever been a female. Henry became infatuated of Anne Boleyn, a noble woman of middle rank. He tried to get Pope Clement VII to annul his marriage with Catherine in order to marry Anne Boleyn but since the Pope refused, he decided to break away with the Catholic church and start the English Reformation.