Act of Union
The Version for English People
These had previously been part of a merely de facto personal union caused by both countries having the same kings - namely the Stewart dynasty.
To avoid "North Britain" at some point choosing it's own king who might even, God forbid, be a Catholic, the English parliament decided to apply some political pressure on the ruling classes in Scotland to make a more solid union.
The biggest problem facing said Scottish nobles at the time was that they and the country itself were skint, having lost a massive wodge on a stupid and ill-fated plan to colonise Panama. So much had been lost, in fact, that Scotland was essentially bankrupt.
So, the English agreed to cover all debts that had arisen from the Darien venture, to the tune of £398,085 (and 10 shilllings)*. In return, Scotland would join the United Kingdom - their national debt being covered by their partner in the new country (i.e. England).
After a certain amount of, ahem, "compensation" for those who would lose out on jobs with the union, it was democratically voted for by both the English and Scottish parliaments and passed into law.
*Of course, this was back when £398,085 was a lot of money - half the entire currency circulating in Scotland. Rather than a nice 4-bedroom semi in Ruislip.
The Version for Scottish People
"We were bought and sold for English Gold, Sic a Parcel of Rogues in a Nation." - R. Burns