It's a question of plumbing. Our hearts have two inlets, one from the lungs and one from the rest of the body and two outlets, again one to the lungs and one to the body. If the heart was situated centrally and all these inlets and outlets were symmetrically placed on top of each other we'd need a bump in the middle of our ribcage to accommodate all the pipework. We tend to be wider than we are deep, however, so the natural solution is to twist the heart round so that the plumbing now fits side to side. So we end up with a right and left side to the heart and the length and position of the major blood vessels determines where it sits. It's actually quite central; it's not way over to either side, but for the vast majority of people this turns out to be turned to the left. As kempie has pointed out, for some people it's the other way round. I'm not sure whether this is a case of DNA or of an 'accident' in development.