The first, and most obvious reason to avoid war is to avoid the massive loss of life that always results from it. This loss of life is never confined to the combatants themselves; there are always unintended casualties.
Another reason to avoid war is that wars are not always the will of the populations that engage in them. War is often a political decision, based on political goals, and not a matter of national survival.
Finally, war often leads to unintended consequences, and are very difficult to stop once they begin. We have been fighting in Afghanistan in one way or another for over ten years, and it's highly possible that when we stop things will revert to the way they were before we fought and gave up American lives.