John Keats wrote "Ode to a Nightingale" during the Regency era, but his work became popular in the Victorian era—a time when England was fiercely divided religiously and politically. Wars, revolutions, and political turmoil in France, America, and Ireland added to a tense national state. Also, the industrial revolution was beginning to pick up—creating poor working conditions and air quality in large cities.
It is easy to see why the narrator in "Ode to a Nightingale" longs for the past—the present he inhabits contains pollution, death, and division. Romantic poets like Keats wanted a simplified past: Roman and Greek culture. We see this in Keats's allusion to Hippocrene—the fountain of the muses—and Bacchus, the Greek god of wine.
Keats also glorifies nature imagery in "Ode to a Nightingale." He wishes throughout the poem to be the bird whose haunting song he hears. He imagines wine boiling from the earth, "Tasting of Flora and the country green." This longing likely stems from the destruction Keats saw in industrial development.
We can also see that religious concerns on Keats's mind in the poem when he describes the "sad heart of Ruth." This biblical story describes a woman who defies societal expectations of marriage. Perhaps Keats admires the sorrowful perseverance in rejecting the status quo.