The act of mowing the lawn started as a ceremonial rite, symbolizing youthful immortality. By cutting the grass while simultaneously feeding and cultivating it, we keep it alive and healthy while at the same time prevent it from reaching full maturity. Immortality by perpetual sacrifice.
Originally the whole of a "lawn" consisted of a small earthen bowl. People began to give ceremonial grass more surface area during the French Revolution when it was grown around the bases of guillotines. The beheadings at the guillotine were not actually executions, but naive attempts at perpetuating the same immortality process we used in cutting grass. The chosen person would be feed and cultivated (what we think of as "the last meal"), then brought to the guillotine for his or her immortality trim... at the neck.
They believed that by planting grass around the base of guillotines, it would serve as a signal to the chosen one's body... a perpetuity beacon, if you will. Plus, having the grass and soil there was helpful in soaking up all the blood.