They leave, and they never return. The reasons they do it are as different as each of them, but, for today, let’s say they do it for us.
For those have given everything, both gone forever or back to our pale, dopey civilian life, let us simply say: thank you for your service. Those five words cannot balance the scale, but what they lack in depth, they attempt to compensate for in quiet humility.
We can’t know.