The Persian Achaemenid empire was founded from the Persis region in 550 BC. It spread over Western Asia, before it was conquered by Alexander the Great by 330 BC. After Alexander's death, one of his general founded the Greek Seleucid dynasty and empire. This empire was then defeated in 247 BC by the Parthians, an Iranian nomadic tribe, who formed their own empire. Finally, this empire was defeated in 224 AD by the Persian Sasanians, themselves also from the region of Persis.
In Persis, the Sasanians were living surrounded by the ruins of the old Achaemenid empire, constantly reminded of the heights their ancestors had reached before it was snatched away from them. In particular, the ruins of Persepolis, the ancient Achaemenid capital located right in Persis, are particularly impressive. Since it has been abandoned centuries earlier, the city has been left untouched by anything but time. All those who walk among its walls are surrounded by extremely imposing sculptures of divine figures and sculpted reliefs representing the ancient Persians being victorious over the rest of the world and all defeated people giving their homage and treasures.
Living surrounded by those ruins had, of course, a profound impact on the Persians. First, this was a constant reminder of what they had once done and could do again: conquered and rule over all of Western Asian. Second, this served as a direct proof of the legitimacy of their claims over that region. The Parthians were only Barbarian interlopers that ought to be put back in their place. Third, the excessive scale of the ruins and their richness perfectly reflect the god-like status of the ancient Persians. As, the Sasanian dynasty claims direct descendance from the Achaemenid dynasty, the ruins are a not so subtle indication of their own godhood, reflected in the title worn by the king of kings, "whose lineage is from the gods".