My theory is that "romanticism" (small r intentionally) is first and foremost a state of mind and a penchant of the heart, a psychological predisposition, a forma mentis, a Weltanschauung if you will which is rather inborn and little, if at all, dependent on external factors. One does not become a romantic, one is born as such; the place, the time, the general stage of civilization and the milieu one is born into can surely stiffle or encourage the latent romanticism but they can neither produce nor extinguish it altogether. One does not even have to be an artist or philosopher in order to be a romantic.
I submit to your consideration the following quotes, which imho are as good a romantic / Romantic ars poetica as anything coming from the pen of Victor Hugo or Berlioz
Et quod nunc ratio est impetus ante fuit. - Ovid, Remedia amoris, 10.
Si vis me flere, dolendum est primum ipsi tibi. - Horace, Ars poetica, 102.