There were actually several reasons why did the war of 1812 started. The official reason as made clear in the declaration of war by the United States of America to Great Britain was the Impression Issue that was made by the British Government under their Orders-in-Council. This Impression Issue is an act whereby men are forced by the government for induction into military service.
Great Britain during those days was already fighting in another front, against Napoleon Bonaparte of France. Great Britain was fighting alongside Spain and Portugal against the Army of Bonaparte who were trying to control the Iberian Peninsula which is composed of Spain and Portugal. Although Great Britain has the biggest navy in the world at that time, the pay and living conditions of their sailors were harsh compared to sailors working on vessels of other countries.