Personal rule, also commonly known as '11 years of tyranny', began in 1629 when parliament forcibly passed The 3 Resolutions by holding the speaker down so he couldn't dissolve parliament. The 3 Resolutions condemned Arminianism, Laudianism, ship money and those who paid ship money. Charles was fed up with parliament not cooperating with him so he decided to rule without it.
For 11 years he collected revenue, conducted religious affairs and (some would say quite effectively) ran the country without consulting parliament at all.
Charles was superb at collecting revenue during personal rule but it came at a huge political cost. He alienated himself from the governing class in England and because of this ruined the possibility of any good relationships between himself and the political nation.
Contrary to popular belief, Charles didn't call parliament in 1640 because of the state of things in England. The truth is, many contemporary views of England during the period of personal rule were that England was being well governed, well run and in a stable environment. Charles called parliament and broke his period of personal rule in 1640 mainly because of his cock-ups in Scotland. Scotland was ready for war with England.
The Scottish were getting a little aggravated with Charles and Laud because they were interfering with the Scottish Kirk and introducing undesirable things such as Bishops and the Anglican Prayer Book. A famous incident that sums up the majority of opinions in Scotland was that of the famous 'stool throwing incident'. It's said that during a service and woman leaped off her stool and yelled 'the mass has come amongst us' whist proceeding to hurl her stool towards the front of the Kirk.
This was pretty much the attitude of all Scottish at this time. They were sick and tired of being pushed around by Charles. He was English! They were Scottish!
Laud, serving to inflame matters even further, issued new canons to the Scottish Kirk. The following was to be read out at church:
"The most high and sacred order of Kings is of divine nature... a supreme power is given to this most excellent order by god himself... Kings should rule and command all persons of rank or estate soever..."
You can imagine the uproar.
In 1638 the Scottish National Covenant was drawn up, it rejected Laud's canons and the Anglican Prayer Book. Eventually it opened the way to thoroughgoing Presbyterianism - it was, however, vague enough for everyone to be able to sign it as it didn't specifically outlaw Bishops.
To Charles, the Covenant spelt open rebellion.
This was not a good look for Charles, he needed to raise an army if he was going to keep the Covenanters in check and to raise an army you needed money. Something of which he had very little of due to the lack of grants from parliament (that, of course, hadn't been called in 9 years) and the threats of tax-payers strikes due to the unconstitutional collection of ship money.
In 1638 the Scottish National Assembly abolished the High Commission and removed the bishops. An army began to be raised. The difference between Charles' army and the Scottish army was that of experience and skill. The Scots army was hardened to war and had previously been fighting in the Thirty Years War with Spain and France, whist Charles' was made from 'perfect militias' - a poor excuse of an army under any circumstances.
It was a simple issue of finance; Charles needed money to fund his war with Scotland. His military costs for 1640 were around 600,000 pounds and when the City of London refused him a 100,000 pound loan he knew he was in trouble.
Charles I called parliament in 1640 not because he'd suddenly changed his mind and favoured a Crown-parliament relationship, not because he considered personal rule a mistake, but because he had no choice. Parliament was the only place that could sufficiently fund a war to reassert royal authority in Scotland.
This series of essays (Decision to..) were written in preparation of exams in a weeks time. Wish me luck. Please feel free to /msg me with suggestions of what I could add, remove, edit, etc. Anything that is inaccurate I'd appreciate it if you gave me your opinion :) Special thanks to iamkaym for your comments.