Charles believed in the "divine right of kings", that

Charles believed in the "divine right of kings", that he was answerable only to God, and
that no mortal subject should question his judgment. Charles wanted no one to question
what he did especially if they found faults in his decisions.
"It is for me to decide how our nation is to be governed, how my subjects are to be ruled and
above all how the Church shall be established under the rule of law. These are the Divine Rights
of Kings and are ordained by the Almighty (God). It is not the place of the subject (citizen) to
question the royal prerogative." – King Charles I of England & Scotland
Rather than working with Parliament, Charles instead collected money for his expensive
foreign campaigns from nobles and knights, arresting anyone who wouldn't pay what he
demanded. When Parliament condemned this arbitrary (random) taxation and imprisonment,
Charles dismissed the Parliament for their revolutionary remarks.
However, when he tried to introduce religious reforms in Scotland, he was met with
resistance that lead to the Bishop’s Wars in 1639 and 1640. Charles was forced to call
Parliament back into order to raise more funds for the war.
Since Parliament helped Charles get the funding he needed, Charles had to agree to give
them more power. They were allowed to reconvene (meet & reorganize) every three years
without being called (by the King), and could not be dismissed without their own consent.
Cromwell had become a staunch (very strict) Puritan after a self-discovery that lasted most of
his adolescence. He believed that he had been a sinner, but was now one of God's "chosen"
people on the Earth.
Cromwell wanted Puritan reforms in the Church of England. Many religious leaders as well
as the King rejected these reforms. The King especially rejected this idea since he felt that he
had divine right and no one should tell him how to run the church. Cromwell felt that
Parliament and the King should be equal partners in English government.
Cromwell was promoted to Colonel in February 1643. This gave him the authority to recruit
and train his own regiment of soldiers. He insisted on strict discipline, which then allowed
his troops to be in more control of their actions on the battlefield, something that the
Royalists could never manage.
Cromwell saw that Charles was not trying to work with the Parliament on important issues.
So Cromwell would draft legislation to limit the power of the monarch. This action causes
Charles to dismiss this Parliament, only to call them back again during the Bishop’s Wars.
Tension continued to mount after the Bishop’s Wars as Charles tried to arrest members of
Parliament who opposed him. Once again Charles dismisses Parliament, when he is unable
top arrest certain members. Cromwell would form an army and lead members of Parliament
into a war with the supporters of the English Monarchy known as the English Civil War.