Virgil, like Homer, gives his characters epithets, which often precede their names. Achates is always faithful, and Aeneas is always pious, even when he is not obviously displaying this particular quality. The Latin word pietas is not precisely analogous to the English word "piety," but it certainly has religious overtones. It means doing your duty to the gods, your country, your parents, and the spirits of your ancestors.
Pietas make Aeneas far more purposeful than any of the Greek heroes, with the possible exception of Hector. Throughout the poem, he is driven by the commands of the gods. The episode that most clearly displays this is one which modern readers often consider discreditable: his desertion of Dido in book IV. It is certainly arguable that Aeneas's initial relationship with Dido is a failure of pietas, since he knows he cannot stay with her. However, when he is reminded of his mission by the gods, he promptly deserts Dido, sacrificing his own happiness and hers, as well as her life, in the line of duty. Aeneas is not to receive any particular benefit for his sacrifice. He will never see the city of Rome. The great Roman heroes, like Lucius Junius Brutus, were distinguished by their ability to put aside personal feelings in the service of their country. Aeneas, however, is abandoning the woman he loves for a country which does not even exist yet, one of the strongest possible examples of pietas.