Romantic poetry was in large part a revisiting of Greek and Roman (thus “Romantic”) culture, philosophical literature, and moral values. This passage, and much of the whole poem, refers the (classically educated) reader to the differences between such philosophers as Socrates and Plato, his student. In this passage, Shelley is alluding to a major difference between them, namely that while Socrates avoided life’s emotional “ups and downs” and sought meaning in logic and rhetoric, Plato allowed himself to feel; he had a young male “lover” Aster (“star”), probably a “Platonic” relationship. In Shelley’s view, this difference was “life-affirming,” the theme of the poem.
The modern student may be forgiven the confusion (a good footnoted edition helps), but the passage is a good reminder that Romantic poetry is not all daffodils and natural vistas, but a response to the over-formal 18th c. verse of Alexander Pope and the like by learned (not naïve) poets, who retreated from city life after their education.