Firstly Scotland didn't join the United Kingdom, no country has ever joined the United Kingdom. Scotland united with England creating the Kingdom of Great Britain. The United Kingdom would be formed when Ireland united with Great Britain.

The two united out of mutual interest and benefit. From 1603 England (and Ireland) had the same monarch as Scotland. England had long been a rival nation to Scotland, but since the Union of Crowns this had largely thawed, and England, as a growing power in Europe, wanted to guarantee Scotland's allegiance and prevent them becoming hostile (ie the Auld Alliance with France).

Scotland did unite for monetary purposes. They poured huge amounts of money into trying to create a colonial empire like the Spanish, French, and English. This obviously failed, and Scotland was in a very precarious economic position, luckily Scotland's monarch is the monarch of England, and England is ready to support Scotland and open up the colonial markets to them. Given the mutual desire of England and Scotland to see guarantees from the other it was the best time to negotiate some form of treaty.

The idea of a treaty soon became negotiations for union. The monarchs since 1603 had often pursued uniting the two (which briefly succeeded under the republican Protectorate). As Scotland would be guaranteed equal status with England in colonial markets, as well as economic support, and Scotland guaranteed to have the same line of succession as England and to be an ally, it became clear to the two to unite.

EDIT- Ooh, my first gold. Thanking you kindly.