Fate and loneliness
As Aldous Huxley famously observed in his Doors of Perception, mankind is doomed to loneliness. Although these lovers felt something close to perfect union, they were separated from each other by cruel fate. The imagery of experience is one of desperate longing for something that never comes. Notice that the poet is perfectly unable to encourage the lover; the lover is convinced that he is doomed to miss his best friend and soulmate forever.
Sleep and dreams
But there's an issue with the lover's loneliness. It belongs to the day time. For eight hours a night or so, he isn't himself. He isn't psychically tied up in the affairs of his daily life. Where does he go in his sleep? That's where this important imagery comes in. The sublime, twilight imagery of sleep is the domain of Morpheus, the god of dreams, who bestows on this perfect sufferer a divine dream of a realm beyond the reality of waking life in time and space. Sleep is a sign of the infinite soul that the body is inflecting. The dream proves to him that secretly, he and his lover are soulmates forever, no matter what life—or death—might suggest.
Water and love
What does the Fountain of Love contain? It contains love, of course—but more literally it contains water. This means that the imagery of love is one of water, and the dreamer encounters this water not in the well, but in his own soul, in his psyche, through dream. The fountain is himself, and the love within him that causes him agony is its own solution; by sleeping, he is dissolved into that water, and in the water of love, he encounters his soulmate again, in his own soul, as if he and her were one single pond of water. The water imagery points to the eternality of love, like still waters that exist forever.
The sublime and eternal
This poem is not just a nice love song. The poet was in his last years of life, and had written literally hundreds if not thousands of songs, often religious songs. He was severely religious, because after all, many of his loved ones were dying each day from the pernicious spread of the Black Plague. This is a poem designed to suggest a response to death and suffering. He communicates this intention by invoking imagery from the sublime: we meet with Greek gods; there are mythic dreams of the eternal realm; there is the mystic union of opposites, and all this for what? Because of the pain caused by fate. This poem is not just a love song, it is a response to human doom and death. It is a religious text, in some senses, if the symbols are seen for their sublime quality.
