Younger people are less certain about what they want to do, and are changing more rapidly. Women are much more likely to quit or start work for family-related reasons. According, more young people and more women typically lead to a higher natural rate of unemployment. Similarly, more highly educated workers change jobs much less. Being more specialized, they have probably already found a good match.
Regulation can greatly increase the natural rate.
Fun facts on gross versus net job creation. Focusing on effort instead of result. If people figure out a way to accomplish the same result with less labor, this means that there is more labor to accomplish some other goal. Partly, this is just a special case of the broken window fallacy, of measuring wealth by inputs rather than output. Saving one person's job may make that person better off, but it also means wasting valuable labor.