Mismatch in Job Market
One smart explanation for the existence of persistent unemployement in the industrial societies, which I beleive is a true one, is the idea of mismatch between the quality of labour supply and the demand by the firms. The theory says that the labour force in modern world needs lots of education and can not adapt herself very quickly to the requirements of the market.
Think about current situation, there are thousands of unemployed IT professionals in the Europe while there is high demand for jobs like nursing at the same time. An IT technician is not able to learn the practice of nursing in a short period of time and therefore she can't take of jobs offered in the other sectors. As a result we see the extra supply of labour in one segment of the market (IT) and the shortage in the other one (Nursing).
This is a structural problem caused by the information lag and delays in the system and seems not to be easily removable. The reason is simple: when one sector is growing it takes time for the educational institutions and the future job seekers to realize it and therefore the familier cycles of supply happens, this time in labour market.
In Iran we are facing almost the same issue: There are many people with higher education degrees who are not effective or qualified enough in one hand and lots of unresponded requirements from the firms especially in the high-tech and oil/gas sectors on the other hand. It explains to some extend the huge wage gap in diffrenent sectors.