Why did the roaring 20s happen? what caused it?

WWI allowed much profit for people during and after it was called ''War profiteering''. This was trickled down. Too many people were offered good paying jobs just like after WWII. Changing political environment in Germany after WWI (with abdication of the Kaiser and loosening of regulations for businesses) allowed them also to have a short ''roaring twenties''  of their own for awhile. That was until the Great Depression stepped into gear, and political turmoil again followed there and elsewhere.

Most likely an increased sense of nationalism brought on by the end of World War I.