Morpheus, the god of dreams
The most obvious symbol in the poem comes when Morpheus is mentioned or invoked by the poet. The setting is this: The lover is unsure what could ever happen that would help him deal with the fact that he and his beloved cannot be together—their fate is sealed, and although the lover wants to hope that one day, fate will bring them together, he's pretty sure that won't happen. Then, Morpheus, the Greek god of dreams bestows a holy vision on him in his sleep where he gets to be with his beloved in their dreams. Morpheus builds a bridge between them in their dreams. This makes the act of dreaming into something deeply meaningful and prophetic, and this revelation helps the lover to heal his frustration by reframing his understanding of reality itself.
The Fountain of Love
The vision comes to the lover in his sleep, as noted, but that doesn't happen just anywhere. He is sleeping beside the titular Fountain of Love. This fountain is a symbolic reference to an infinite well of love. If the love that they shared healed them of their frustration, before they were separated, then an infinite source of that love could be enough to redeem the whole of reality (that's what the poetry suggests). It is from the real fountain of love, the unconscious, that dreams come from that heal this poor man. The fountain of love was within him the whole time.
The beloved
This person is a symbolic character, because she isn't really in the poem except for in the soul of the dreamer. So, what is she? She is the archetype called the "Anima" by Carl Jung. This mystic female represents the entire process of love for the soul, because love is the only thing that makes life worth living (which the lover observes in blatant language). The beloved is a sign of the soul's need for meaning and love. The union with her would mean healing and redemption for the dreamer, so when they meet in the dream, that is important symbolically.
Sleep as the soul
The pain of loneliness is an affliction that the lover has to endure in his waking life. In his waking consciousness, he is himself. He has a body. He has a point of view, and he has a fate—a painful fate that removed him from his soulmate. When he sees her in his sleep, that is his descent into his soul, and so his soulmate can meet him there. Sleep is a symbol in this poem, therefore, because it is the fertile domain of a new kind of experience—not one of waking conscious, but a real soul awakening. The poem suggests that perhaps the dream is the reality, and waking life were the dream.
The mystical union of opposites
The man is real, and he exists in the material world, but when he encounters his beloved, she is unconscious, unreal, and immaterial. This is like a yin-yang union of the opposites, because male and female are united by Morpheus's divine dream, and so are consciousness and the unconscious, day and night, real and unreal. This hints at an evermore important union of the opposites: the union of life and death, the promise of reunion in the afterlife. Perhaps, their souls are permanently entwined by fate, and their lives are simple a narrative where they learn to long for one another before, in death, they are permanently united. That is the symbolic reading of the poem as a response to existential suffering (which is the proper way to read the poem, one might argue, since the poet wrote it late in his life, after many of his loved ones died from the Black Plague).
