Julius Caesar goes in great detail about his reasons for going to war in his book "Commentarii de Bello Gallico". As it was written by Caesar himself, it may be a bit biased, but at least it gives a insight into how the Romans viewed their reasons for going to war. Conquest in itself is seldom the reason for war used by Julius Caesar, and the Casus Belli used by Julius Caesar can instead be divided into three different types:

1: The Barbarians (to use a Roman expression) threatened Roman Lands, either because of a planned invasion or by attacking allies of Rome. Because of this threat the Barbarians had to be pacified, and became a vassal or part of the Roman Republic after the war.

2: Preventing a coalition against Rome from being formed.

3: Putting down rebellions against Rome, which included allies (Outside Roman control) of the rebelling states (That were under Roman control). The Allies would then be pacified after the war.

So basically most of the Casus belli were about protecting land already part of the Roman Republic, or preventing invasion into it.