> The Republic had a culture that very much treated war as a glorious thing, and mobilized the public for total war, over and over again.
I think there are two big differences.
Early on the legions were on “Team Rome”. Scipio Africanus and Asiaticus conquered large regions but never turned their armies against Rome. Sulla turned his armies against Rome. As did Julius. Legions were seen as more for the glory of a general than for the glory of Rome.
Second, conquering new territories was a way to increase a person’s prestige relative to their peers. So there was a kind of friendly competition with various consuls trying to outdo each other.
The Empire changed everything. The Emperor already had prestige relative to his peers. Trying to conquer new territories was a high risk activity for not that much upside (you were already emperor). For example, Augustus knew that Crassus (died at Carrhae) and Mark Antony(defeated in Parthia) had huge setbacks that undermined their position. Even Augustus suffered the disaster at the Tueteborg, but through a lot of PR effort was able to pawn it off on Varrus who conveniently was not a part of the immediate Imperial family.
The other danger was that if there was a victory, it might be enough to propel the commanding general to rivalry (see the later example of Vespasian and the Jewish rebellion). Augustus was an brilliant politician, but not that great of a commander, and had to rely on others (see Agrippa) for actual battlefield command.
Thus given these risks Augustus was not very aggressive about expansion (though he did conquer northwest Spain, and his armies made some expeditions in Germania).
Given that he was the first Emperor and ruled so long, he kind of set the precedent.
EDIT:
It is interesting that instead of launching a punitive expedition against Parthia to retrieve the captured Roman standards from Carrhae, he recovered them through diplomatic means.