A good question! This issue has been hotly debated in the second half of the last century, but the dust has now mostly settled. Before we get to an answer, we must cover the history of Roman wars.

In the early days of Rome, back when it was still a city state under the kings, Rome was situated in among other cities and tribes, which we collectively call the Latins (named after the region in Italy called Latium). There appears to have been quite some politics going on between these cities, with regular fighting (similar to that of city states in Greece). Rome was often involved in these wars, either as a principal participant or intervening on behalf of one side or another. These cities however also formed a coalition to defend against outside threats, such as the Etruscans and roaming Gauls. Historians call this coalition the Latin League. With military force, both in the city-state wars and the coalition wars, the kingdom of Rome secured a leading position in the League.

After the kings were deposed, the fledgling Roman Republic stayed involved in the Latin League. Many Latin cities were dissatisfied with the dominant position Rome commanded (or demanded) in the League, so in the first years of the Republic a major battle was fought between several Latin cities and Rome. According to Roman tradition, the Romans were victorious and dominated the Latin League for the next century or so. It is not clear that this was actually the case though. It's likely that Rome was either an equal partner of the Latin League (not in it but allied with it) or 'just' another member of the League.

What we do know is that during the 5th century BC Rome and the Latin League had to fight regular wars fending off Etruscan and Itallic enemies. Early in the fourth century Rome captured the Etruscan city of Veii, greatly increasing their stature and power in Latium. This victory was however soon overshadowed by the sack of Rome by Gauls in 390 BC. This reversal of fortunes led several Latin cities to no longer support Rome via the Latin League. Indeed some cities seem to have switched sides to enemies of Rome. Meanwhile Rome and its loyal Latin allies kept fighting regular wars with the Etruscan and Itallic cities.

All this time Rome was still a quite small city state, engaged in the to-be-expected city-state politics and wars. From 366 BC however, Rome started to expand in central Italy. Pretty much every year one war or another was fought with the Etruscans, the Latins, the Samnites, the Umbrians, and so on. In this long series of wars lasting from 366 to 266 Stephen Oakley identifies two turning points:

• First, the end of the Latin Wars in 338. Rome's Latin enemies had been defeated; Rome's dominance of Latium was now absolute. The Latin League was disbanded, with Latin cities being incorporated in the Roman state (if they had behaved badly) or being left as an 'independent' state - surrounded by Roman territory - (if they had remained loyal).
• Second, the battle of Sentinum fought in 295 deep in Umbria. In this battle Rome defeated a coalition of Etruscans, Samnites, Umbrians and Gauls. It was the most powerful coalition of cities and tribes Rome ever faced. After their victory in this battle, Rome had secured its position as the most dominant power of central Italy.

From here, Rome conquered the rest of the Italian peninsula. Later in the third century Rome came into conflict with the Carthaginian empire, beginning its expansion overseas.

It is interesting to note that in one century Rome went to war for 95 years. Fighting had a cyclical nature, with new legions being raised in spring, fighting during the warring season, and returning home before the end of the year. Why did Rome fight so much and so often?

For a long time, the case was clear for historians. Rome was fighting to defend itself and its allies. Those damn Etruscans and Samnites and Gauls kept attacking and Rome had to do something. This notion has been called 'defensive imperialism' and it really got started in the second half of the 19th century. It is no coincidence this happened then, as the 'defensive wars' of Rome mirrored the wars of unification in Germany and Italy. If Rome was justified in fighting for the unification of the Italian peninsula, surely so were Germany and Italy justified in fighting for their unification. The German historian Theodor Mommsen argued that Rome was merely fighting to maintain the status quo; that Rome wasn't driven by a desire to expand, but that it was a case of conquer or be conquered. Note that in this view Rome acts much like a nation state: a single actor, acting rationally. 'Defensive imperialism' was the main school of thought among ancient historians throughout the early 20th century. Look for works by Maurice Holleaux and Tenney Frank to find nice examples.

In the second half of the 20th century, things began to change. The Western empires had fallen or were in decline. There no longer was a need to justify these empires using the ancient Roman example. The main view even went the other way. As the expired modern empires were viewed more negatively ('imperialism' became a bad thing instead of a good thing), so too was the Roman empire [I'm using 'Roman empire' here a bit imprecise, referring to expansive Roman Republic, which in this view behaved like an empire, instead of the - later - Roman Empire proper).

At the end of the 1970s William Harris published a book, War and Imperialism in Republican Rome 327-70 BC, in which he argued that not defense and security were behind Rome's warmaking, but social and economic factors. In short, war was profitable. It led to glory and power, to riches and profits. This view quickly gained prominence, but not without debate. For one thing, it was criticised as being too single-minded just like the 'defensive imperialism' notion.

Despite it's single-minded nature, Harris' book was welcomed by historians for its novel approach and its thoroughness, and it sparked a debate. In that ensueing debate some historians defended defensive imperialism, some historians defended Harris, and some historians came with newer still analyses. The most important of which was Arthur Eckstein's Senate and General: Individual Decision-making and Roman Foreign Relations 264-194 BC.

Note that both defensive imperialism and Harris' aggressive imperialism assume Rome to be single actor. Rome fought wars, Rome annexed areas, Rome made decisions. Granted Harris' found that societal and individual factors (economic benefits and glory) played a role, but he saw that as defining the Romans, who acted as a single group. Eckstein changed this, as should be clear from the title alone: it's about individual decision-making. Let me quote from a post I made some time ago:

Eckstein examined many of the wars in that period. He looked at the instruction generals (consuls or praetors) received from the Senate and what the generals actually did while in the field. He found that the generals had a large amount of freedom to do as they saw fit. Attacking cities, making peace treaties, provoking wars, it all happened. In theory all actions of the general had to be approved by the Senate at the end of the year, after the general returned to Rome, but Eckstein found that this was just rubber stamping. In all those years, it happened only once or twice that the Senate did not approve.

The freedom Roman generals enjoyed can hardly be overestimated. Roman consuls and preators were granted imperium, which historically was the power of the kings of Rome to act in the public interest. When the Romans deposed their last king, they didn't want a single person to wield all that power, so they divided imperium among two yearly elected consuls. Later praetors were added, who were granted a lesser form of imperium. The power of consuls was only limited by the veto of their collegue or of a tribune of the plebs. Once a consul was in the field with his army, he could wield his power nearly without restraint. Eckstein found they often used this power to the maximum extent. Some consuls even went looking for wars outside of the mandate they were given by the Senate at the start of the year. It is cases like that which show the idea of Roman expansion being a conscious, established aim of "Rome" to be false. Sometimes it was just individual generals being greedy for gold or glory.

With wars providing opportunity for gold and glory, individual senators had cause to procure positions in which they or someone they're close to would get these opportunities. Rival senators would have attempted to thwart that though so that their own candidate would be elected to such a position. Because the election of consuls and praetors happened seperately from the assignment of 'provinces', it could happen that someone would secure a consulship but that he would get assigned an area in which nothing was to be won. The practice of extending command of armies to ex-consuls for another year (prorogatio) made this trick especially popular.

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