Explanation: Romans had a weird relationship with peace. Unlike many warlike cultures, they did not glorify war simply as a means of destroying outgroups and enriching the in-group (though they certainly destroyed many others and enriched themselves in the process, such was not the stated rationale of Roman wars, generally). Instead, the Romans envisioned their rule as bringing peace to far-off lands, who would be better under their (undoubtedly enlightened!) rule.

The fact that Rome spent less than 20 years of their ~1000 year existence at peace seems to have been a minor detail.

This outlook has some strange (and interesting) effects within Roman culture - Roman culture often adapted the personalities and mythologies of the Greek gods to their own, but in the case of the gods of war, Ares (Greek) and Mars (Romans), there was a very notable difference. Ares was a brute, an agent of chaos, a destroyer; all the things the Greeks saw war as. Whereas the Roman Mars was a protector, a god of forests and agriculture in addition to war (though really, almost all Roman gods were also gods of agriculture), a god of discipline and order; all the things Romans saw war as meant for.

Or in the words of a Roman poet…

Roman, remember by your strength to rule
Earth’s peoples—for your arts are to be these:
To pacify, to impose the rule of law,
To spare the conquered, battle down the proud.

Of course, while how nations view themselves is important to how they act, it also is often very far from the whole truth. The Romans were certainly also as treacherous and opportunistic as they come - constructing treaties with Catch-22s and forcing impossible conditions in negotiations in order to press their military advantages against weaker neighbors, conquer, and thoroughly loot them!

Germans: hey stop killing and conquering us!

Tiberius: “…and I took that personally”