There are several different perspectives on this. First of all, the Roman society was essentially a society using extensively slave labor as the most important form of labor in existence. This basically ranged from constructions to simple chores around the house and often to farming as well, entertainment of its citizens and in other battles. A society relying so much on slaves for its own economic benefits could only necessarily force wars and battles in order to constantly keep a thorough supply of slaves available for work.

Indeed, in general, the population of a nation that had been defeated in battle would have either perished in the fights or would have been enslaved. Enslavement meant not only work in the city of Rome (or elsewhere in the empire), but also the possibility of being sold in exchange of money or products. What this virtually meant was that the slaves became a source of richness themselves.

Further more, the economic necessity of war also came from the economic added-value wars provided, even besides slaves. For example, Dacia was invaded by Trajan solely because of its enormous deposit of gold. It was essentially, a looting campaign, despite the fact that the country was then colonized (however, this lasted only until 271). This is not a singular case, Gaul was also attacked on similar looting grounds. As we can see, the economic motivation implied the necessity to wage war on other rich nations.

Perhaps a reasonable explanation for this lies in the limited capacity of the Ancient world to produce wealth outside a country. As such, despite the fact that commerce was extensively used during that time, a nation could not solely rely on that for its survival. Generally, it was either national production or, in the case of stronger states, looting campaigns. Basically, expansion and, thus, war was a possibility for the Romans to gain new resources.

Further more, especially towards the end of Rome's existence, but also before its affirmation as a world power, there was a very strong incentive of the need to show to...

The incapacity to show this would have naturally led countries to revolt against Roman leadership and to overthrow its governance. In Antiquity, a power was forced to show the other competing entities that the respective nation was stronger than any of the other and that it had the power to crush virtually all potential rebellions.
This was also the case with Rome: any sign of weakness, manifesting itself in the incapacity to wage wars on its opponents, could be interpreted as a sign that Rome was not capable of facing its enemies and would have meant its extinction. This actually happened in the 5th century, when war was no longer able to successfully wage war, bringing about its decline.

In the end, we also need to point out that this was essentially a very brutal and violent society and where survival could only be ensured by war. Historians report of entire cities being wiped to the ground (including Carthage), with the entire populations being slaughtered or, in the best case, sold into slavery across the vast empire. Such a violent society virtually encouraged war for the survival of a nation and Rome made no exception.

As such, following the argumentation previously presented, we can strongly conclude that, for all political, military, economical or social reasons, war was fundamental to Rome's very own existence and, as such, was a pure necessity during its history. Without a society thus constructed to encourage war and to successfully fight off its neighbors and other regional powers, Rome would have not been able to survive and would have most likely ended up conquered by a third party.

Bibliography

1. Millar, Fergus. Emperors, Frontiers and Foreign Relations, 31 BC to AD 378.

2. Harris, William. War and Imperialism in Republican Rome. Clarendon Press. Oxford Millar, Fergus. Emperors, Frontiers and Foreign Relations, 31 BC to AD 378.

Harris, William. War and Imperialism in Republican Rome. Clarendon Press. Oxford