Why was self sufficiency important to a country practicing mercantilism?

Early mercantilism meant primarily aiming to reduce imports. This issue is related mainly to articles of luxury. The mercantilists believed fact that the foundation of the wealth of the state is a positive trade balance - the dominance of exports over imports. Source of such beliefs lay still in the medieval practice of governance - the monarchs gather financial reserves necessary for an effective policy. However, in the developed theory of mercantilism, welfare of the residents and the strength of the state directly combined with the development of the industry. It is associated with it hopes for economic self-sufficiency of the country. In addition, the mercantilists preached direct relationship between economic power of the country's human potential.