Why are the walls of the left ventricle thicker than the walls of the right ventricle?

The human body possesses a double circulatory system, in which blood passes through the heart in two cycles. In the first cycle, the right ventricle only pumps blood as far as the lungs (to enable oxygenation of the blood). When this newly-oxygenated blood returns to the left hand side of the heart during the second cycle, it is then pumped around the entire body by the left ventricle. This requires significantly more force than the pumping of blood to the lungs, hence why the left ventricle has thicker walls than the right ventricle.