The fleeting nature of happiness and youth is one of the great themes in Keats's Great Odes. In the "Ode on a Grecian Urn," for example, he becomes envious of the people depicted on an old pot, because they always remain in the same constant state of joy. In "Ode to a Nightingale," the speaker manages to imagine himself into such a state: the nightingale's world. But the imagination is not powerful enough to carry on the fiction after the nightingale has flown away, and his waking vision ends after only a few stanzas of bliss.
The end of the poem suggests that life might be just as much of a dream or fantasy as the speaker's "waking dream" about the nightingale.
The flowers in the poem symbolize momentary intoxication and fleeting beauty.