Because the heart is forced to pump out blood. In the upper right side of the heart is an bundle of cells called the sinoatrial node. Unlike any other cell in the body, these cells (nicknamed "pacemakers") keep sending out electrical impulses, more than once a second, to make the rest of the heart contract and pump out blood.
It consist of cardiac muscle tissues, which means it moves involuntarily. That is why your heart never tires of constantly beating to pump blood throughout your body.
