Because Henry convinced himself and tried to convince the church, that marrying Catherine was a sin and soon became obsessed with finding a divorce so he could marry again and father a legitimate son.
By 1531 Henry appointed Thomas Cromwell as his chief advisor who devised a plan to get a law passed that made Henry the head of the Church of England (Act of Supremacy) which allowed Thomas Cramner, the archbishop appointed by Henry, to void Henry's marriage to Catherine and secretly marry him to the pregnant Anne Boylen. Henry and all his successors will be appointed head of the Church of England.
"the King's Majesty justly and rightfully is and oweth to be the supreme head of the Church of England, and so is recognised by the clergy of this realm in their Convocations...present Parliament that the King our sovereign lord, his heirs and successors kings of this realm, shall be taken, accepted and reputed the only supreme head in earth of the Church of England called Anglicana Ecclesia, and shall have and enjoy annexed and united to the imperial crown of this realm as well the title and style thereof, as all honours, dignities, preeminences, jurisdictions, privileges, authorities, immunities, profits and commodities, to the said dignity of supreme head of the same Church belonging and appertaining...shall have full power and authority from time to time to visit, repress, redress, reform, order, correct, restrain and amend all such errors, heresies, abuses, offences, contempts and enormities, whatsoever they be, which by any manner spiritual authority or jurisdiction ought or may lawfully be reformed, repressed, ordered, redressed corrected, restrained or amended, most to the pleasure of Almighty God..."-Act of Supremacy
When Henry became head of the church he cut off all payments of papacy, regulated church doctrine, and closed all catholic monasteries in England; many people were unhappy while others welcomed this church.