The Pax Romana is pretty much seen as the most peaceful time in ancient history. Why's this important? Well from answering the question why it was so peaceful you can see the greatness of the Roman empire. They had utterly dominated any threat to their empire in the preceding years and now had no real enemies.

So why weren't Rome starting loads of wars if they were so dominant? The slowed expansion of Rome in this time showed what they had accomplished, they had a flourishing economy, controlled the Mediterranean and trade into Asia along with the vast grain supply from Africa. So there was no real need other than personal glory for a general to go and conquer more land in the name of the empire. Rome had everything it needed

There were few rebellions in occupied lands which showed they were well governed and that people in conquered lands had accepted they were part of the Roman empire, something most expansive empires have trouble with, (and indeed does become a problem for Rome during the late second century)

I also should mention that this period of peace was held in place by some of the greatest rulers ever, there were some bad ones (especially Caligula) but the work that went before and after them helped keep the Roman leadership intact, when there was peace in Rome there was less chance of an opportunist general declaring himself imperator and inciting a civil war and allowing barbarian tribes to reclaim land left abandoned

Hadrian and Trajan also helped keep the peace, Trajan first crushing unrest around the borders before it could become a real problem and then Hadrian abandoning some of the lands in favour of a more secure border

Tl;Dr : the peace showed the Romans dominated foreign threats, their own people and trade in Europe.