Rome made war to keep its core confederate members together.  The Romans always made political "pretexts" to justify their military actions as "defensive" so that their allies could be brought on board more easily.  Rome understood that if things stagnated, the allies would get restless and seek to go off on their own.  Rome couldn't allow this, therefore, they kept them occupied with war which fed everyone's coffers with booty.

I bet it also had to do with selling the idea of combat internally to Rome's own citizens.  From what I recall the Romans thought that the gods only looked favorably on war that was based on self-defense.  By coming up with self-defense rationales, they created religious justifications that their people could believe in.

I agree with this.  The people always looked to the Temple of Janus during war time so religion was definitely a key component.