From what I can tell, the wars that expanded Rome considerably (between 500-300 BC) were quite complicated. Some argue that Rome was motivated by defense and security while others argue that it was purely to increase power and prestige. I have yet to find a source which provides a complete picture of Roman motives. I would really appreciate some input as I am insatiably curious about this topic.
I have written my Master's thesis on Roman decision-making in the Senate (or more precisely decision-making by Roman senators) during the expansion in Greece and Spain from 218 to 133 BC. It's a bit later than what you're looking for, but much of the background and context still holds true.
The notion of 'defensive imperialism' 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 critisized as being too single-minded just like the 'defensive imperialism' notion.
As glory, power, and profits came into play, the importance of individuals in the warmaking was highlighted. No longer was Rome as a single actor doing something, but it was Roman society, a collection of individuals, valueing these things. This idea was extended in the 1980s, most notably by Arthur Eckstein in his book Senate and General: Individual Decision-making and Roman Foreign Relations 264-194 BC. 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.
This close examination of what generals did shows that you can't really talk of Rome doing something. Once the Senate sent out an army, the general had such a large degree of freedom that the Senate had very little say in what happened. Note that the Senate was fine with this, as all generals were senators, elected by the other senators and if the current general could do pretty much what he wanted, so too could next year's general, which would be another senator. Even if a senator missed out this year, he - or someone he's close to - could be up next. (I found the same dynamic at work in the period 218-133 BC.)
In conclusion I'd like to point to John Rich's article Fear, Greed and Glory: the Causes of Roman War-making in the Middle Republic. The title of the article summarises what I said above, the content of the article expands on what I said above. If you're only going to read one thing, read that. :)
Works mentioned:
Other interesting reads:
[edits for spelling, grammar, typography]
You won't find a clear answer for this because one can't exist. Rome wasn't a single entity with one motive and aim. It was a collective of people, some with diametrically opposed views.
I'm sure you are aware but history (and very much ancient history) isn't about finding a tidy primary source and going 'wow that answers our question nicely'. Evidence is often contrary to other pieces, and evidence which does proport a motive is never as clear cut as its face value.
You weigh the evidence, balance it and try and fit it into a narrative. It very rarely works, which puts us in a position where you could often write two articles based on the same evidence and both be as valid as each other.
So even ignoring the problem that you will never be able to work out a single motive for 'Rome', you wouldn't believe it even if you found it.
I'm on my mobile so can't at the moment but if you're interested I'll try and dig up some of the stuff from a lecture I had on this as an undergrad because you're right, it is very interesting. But don't expect a clear answer, simply look at the evidence and reconcile it as best you can. It is almost always ok for the answer to be 'we don't have enough evidence to know for sure'.