The Roman Empire was only stable on the parts bordering the Mediterranean, where you could quickly and (fairly safely) sail. Gaul, the Balkans, Brittania, etc were constantly rebelling or getting invaded.
Lots of modern europe was never touched by the Roman Empire because it was too far from the water.
I am not a historian, but the Romans seem to have grown at the expense of weaker and more disorganized foes, who either didn't have any form of centralized leadership prior to contact with the Romans (like the Gauls and Iberians) or who were pitched against eachother in civil war (like the Egyptians). When fighting against similarly sized centralized states, like Parthia (or Carthage, in the early republic), they had a much harder time winning anything out of them. I think they mainly exploited a preexisting assymetry.
That's an interesting question I've thought a lot about in the past (the rise of the great empires in general, to be honest).
My best guess is that they were able to do this thanks to the advancement required to face off the threads before their eventual rise. There wasn't anybody who could really compete with what they had after those wars were won. The Pax Romana held relative peace in Europe because of that strength.
I'd love someone with more knowledge on the subject to come and answer this - really fascinating subject.
A lot of Roman strength was organizational. In conquering, they spread that knowledge. It would probably have been hard to do it again with the same technique.
Lots of modern europe was never touched by the Roman Empire because it was too far from the water.