One reason for unemployment is voluntary. Some of the unemployed have saved enough money so they could quit unfulfilling jobs. They have the luxury to search until they find just the right opportunity.
The second cause is when workers must move for unrelated reasons before they can start searching for new jobs. The third reason is when new workers enter the workforce. That includes students who graduate from high school, college or any higher degree. They have more skills than if they didn't go to school. That's a major reason for youth unemployment.
The fourth reason is when job seekers re-enter the workforce. They went through a period in their lives when they stopped looking for work. These include mothers who are rejoining the workforce after their children are old enough. Other reentrants might have gotten married and set up the household while their spouse worked. Other reentrants had to care for elderly relatives before returning to the labor force.
These four situations all cause frictional unemployment. They are an unavoidable part of the job search process. However, the good news is that it's usually short-term.