There's actually a surprising lack of these sort of questions here when I searched for them. Not to discourage someone else a shot but I did come across this thread which may be a good starting point for you. It's not explicitly the same question but the answer I think does a good job of giving you some good information.

Roman expansion has been debated a lot among historians of the ancient world. Did Rome have some kind of grand strategy for its expansion? Or was it perhaps just happenstance - was Rome dragged into war after war without overarching goals, resulting in an empire by accident? In the last few decades a consensus has emerged where the expansion of Roman territory is seen as a complex interaction of social, economic, and political factors.

Back in the 19th century, the view of Roman expansion was very simple: Rome fought just wars, in defense of either itself of its allies. At the very start of the Republic, in the late 6th century BC, the very first war of Republican Rome was apparantly against their former king and his Etruscan allies. Many wars with Etruscans followed. Later, in the 4th century, Rome allegedly came to the defense of nearby Latin cities against the Samnites. Once peace with the Samnites was concluded, it is said the Latins plotted against Rome. Rome responded by declaring war. Many 19th century historians took these claims at face value. Rome conquered Italy in series of defensive wars, and later expansion outside of Italy could similarly be justified (for instance, according to Roman claims the Second Punic War started after Hannibal attacked Rome's ally Saguntum). This view has come to be known as 'defensive imperialism'.

It is no coincidence that 'defensive imperialism' was the main view in the late 19th century. Germany and Italy went through a rather violent unification process, and the fact that Rome had fought just wars for the unification of the Italian peninsula justified the German and Italian unifications of the 19th century. Furthermore, the just expansion of the Roman empire across the Meditteranean justified the expansion of European empires across the world. For almost a century the notion of Roman 'defensive imperialism' held sway.

In the second half of the 20th century the view on Roman imperialism started to change. Note that this is in step with a changing view on modern imperialism (modern empires had fallen or war in decline). Especially in the seventies studies into Roman expansion were popular. They often found that Roman justifications for wars were not tenable, if not entirely fictional. The must have been other reasons for Roman expansion. In 1979 William Harris published a book which would change historians' views on Roman expansion dramatically. Titled War and Imperialism in Republican Rome 327-70 BC, the book took a detailed view of Roman wars and the factors playing a role in going to war. It is hard to summarise such a seminal analytic work, but I'll try to give the gist of the work.

In five parts, Harris analyses various factors:

1) Roman attitudes to war

For the Roman aristocracy, "success in war was by far the most glorious kind of achievement" (p. 11). This led to Roman magistrates - who in this period were elected only a year - preferred to spend their year in office fighting a war rather than being a glorified judge.

2) Economic motives for war and expansion

Harris says: "it was scarcely possible for a Roman to dissociate the expectation of gain from the expectation of successful warfare and expansion. No Roman senator had to convince other senators that victory was, in general, wealth-producing." (p.56)

3) Expansion as a Roman aim

Harris opens this part with a bold statement: "The rules of the Roman state wished to increase the empire, and this was one of the overriding and persistent aims of their external policy". (p.105) He goes on to state that there was no long-term plan or strategy to expand, but there was a consistent drive to expand.

4) Annexation

In this part tries to explain why Rome didn't always actually proceed with annexation of vanquished foes. In short, he believes Rome was satisfied with projecting their power and wasn't really interested in whether or not that was done by annexation.

5) Imperialism and self-defence

Finally Harris tackles head-on the notion of 'defensive imperialism'. Having shown three causes for Roman expansion (parts 1-3) and having shown that a lack of annexation does not mean a defensive attitude (part 4), he dismantles 'defensive imperialism' by having a detailed look at Rome's various justifications for wars. He describes the Roman ideal of just war, before discussing how Roman authors tried to appeal to that ideal by describing the causes for wars in a certain way.

Harris' explanation of the causes for Roman expansion has been criticised as being too one-sided, effectively substituting an 'agressive imperialism' for the earlier 'defensive imperialism'. In his article Fear, greed and glory: the causes for Roman warmaking in the middle Republic John Rich tries to provide a more nuanced view. He notes that Harris is most compelling in his first two parts, where he analyses the structural causes for Roman warmaking. Rich adds to that that there also were limiting factors acting as constraints, such as the supply of manpower and the aristocratic competition. Furthermore, he notes that while most Roman wars were not just per se, some were inspired by fear of an enemy. Sometimes that fear was legitimate, sometimes it was irrational (in hindsight), but it certainly played a factor in Roman decision-making.

To summarise: Roman expansion wasn't "planned out" well in advance. Roman expansion was driven by several factors, the most important of which were: a lust for glory, economic gain, and fear of certain enemies. Some wars were fought in defense of allies, but only if Rome or Roman aristocracy stood to gain from it.