When you lift weights, the smaller cells or tissue in your body are torn and rebuilt. The harm caused to the smaller (your muscle cells/tissue) causes your body (the greater) to grow stronger.
In war, men fight and die. They are destroyed for the (presumable) good of the greater–their nation. The smaller dies for the strengthening of the greater.
Things in life seem to work that way. You give up a small present moment, for a greater future. Sacrifice of the “now” is often painful, but it leads to a more robust and stronger future.