According to Bruni:
15th century Florentines love freedom because the city was founded during the Roman republic.
Founded by veterans of the dictator Sulla, mind you.
If the city was founded just a few decades later, during that terribly oppressive age of the Ceasars, Florence would not have inherited that love of freedom.
Bruni even cites Tacitus, who wrote in the late 1st century AD, as a support for this view.