Rome was in a constant state of war during it's existence, did the Romans usually wait for a justified reason to declare war or did they feel no need to do so?
edit: For clarity
Since a similar question was asked a while back, I have copied and emended my answer from there. Be sure to check out /u/QVCatullus' answer there, too, for an example of how the de facto act of declaring war played into the justification for it.
To say that Rome was in a constant state of war during it's existence is a bit misleading, since there were long periods of peace, too (even though the 'Pax Romana' wasn't necessarily peaceful all the time and everywhere). But that's just a minor nitpick.
It's true that the Romans sought forms of justification for their wars: The theory of the 'bellum iustum', the just war, was quite popular in Republican times. It also means a bit more than a 'justified war', originally it meant more a 'war waged and opened according to the laws'. Cicero expands the whole concept a bit about it in de officiis and de re publica:
but our people have already brought all the world into their possession by defending our allies (sociis defendendis) [rep. III, 35]
This was one aspect of a just war. If you came to the aid of your ally, and beat the aggressor, annexing or pacifying some of their lands was justified. This happened often, since the Romans were in the habit of associating themselves with bilateral treaties with many sovereign entities all across Italy and the mediterranean, and, since Rome was since the fall of Veii and the dissolution of the Latin League the mightiest and most powerful actor on the Italian peninsula, other communities were often looking to Rome for help against aggressors. This was the process which led Rome to control over the whole of Italy over a series of various wars, often not even started by Rome but where Rome intervened in a conflict that concerned one of their allies. Of course Cicero in that above quote also takes a few liberties with the interpretation of events, that the Second Punic War, e.g., was simply waged for the defending of allies is, ahem, debatable.
The Second Samnite War shows an example for how such a war could erupt: Rome had allied itself with the Greek city-state of Neapolis (modern Naples), which was attacked by the Samnites (the earlier First Samnite War, which is probably apocryphal, saw Capua dedicating its territory to Rome to make Rome - bound by treaties to the Samnites themselves - help them defend against the Samnites). This, after a few severe setbacks, led to Rome defeating the Samnites and establishing a bunch of colonies on their land. This process repeated itself often during the early history of Rome, ending with Rome controlling almost all of the peninsula either directly or via treaties.
Caesar, too, claims defense of allies frequently as a justification for war in his commentarii on the Gallic War, e.g. he justifies the first crossing of the Rhine, claiming that the Ubii had allied themselves with Rome and now wanted help against the Suebi (Bell. Gall. IV, 16 1-6). Caesars conquest of Gaul is defended by Cicero, who said that this was necessary to end the permanent Gallic threat to Italy's northern border, and this could only be accomplished by conquering all of Gaul, not only the tribes that directly threatened Rome, which already stretches his criteria for a just war quite a bit by waging a preemptive war.
Cicero also formulated some other criteria for a just war:
Those wars are unjust, which are undertaken without cause. For apart for the cause of avenging <oneself> or beating back the enemy, no just war can be waged [...] There is no bellum iustum without first threatening war, without declaring war and without demanding compensation (nisi denuntiatum, nisi indictum, nisi de repetitis rebus) [ibd.]
So, his criteria are these:
He also cautions that, even if injustices are suffered, revenge must not be inappropriate, the laws of war have to be obeyed, and negotiation takes precedence over force [de off. I, 34f.].
Earlier, the Roman concept of a 'just war' simply involved lawful declaration of war by the fetiales, the priests responsible for foreign relations: The fetiales would demand amends for suffered injustices (like stolen cattle, or land taken), res repetere, and, if denied, would throw an iron-tipped or fire-hardenend cornel cherrywood spear (the hasta fetialis) over the border into the enemies' territory, which formally signified the beginning of hostilities and was also a symbolic appropriation of their land. This type of behaviour was quite common for primitive societies in which cattle theft is the most common form of hostile tribal interaction (think of Homeric Greece). Later, when this wasn't practicable anymore, since the borders had been moved far from Rome, this symbolic act was move to the temple of Bellona. This concept of a 'just war' was not without critique: the Greek philosopher Karneades argued that this was simply a practice used by the Romans to appear lawful while being unjust - this form of declaration of war observed the cultic formula, while violating morals. Cicero follows his critique in rep. III, 20:
How far utility has distanced itself from justice, the Romans themselves show, by having the fetiales declare war and legitimately committing injustices (legitime iniurias faciendo), they have brought the whole world into their possession.
This was at least some of the ideology behind it - which doesn't tell us much about motivations and ulterior motives.