Love
The entire poem is a really just a series of metaphors on a single motif; a motif that is presented in the opening line. (Though, technically, that is not the title of the poem, which is “Song.”) The speaker opens by asking what love is and though this recurring motif answers through a figurative language and symbol. Ultimately, the answer seems to suggest that love is itself merely a symbol of something.
A Flower
The third stanza presents one of the most direct symbols of this recurring motif through the common metaphorical association of love with flowers. The symbolism takes love from a flower that smells sweet to one which lost all its sweetness in the death of an hour.
A Fragile Thing
The fourth stanza confronts the motif by associating love fragility. The fragility of love is compared first to childish behavior and then to a more adult emotional response that is equally fragile: vanity. The metaphor is extended with a striking juxtaposition of bubbles made by rain being firm when, of course, the irony is that bubbles are every bit as fragile a child or a vain person.
Love as Law
The final stanza presents what may well be the most jolting addition to this motif of musing on the subject of what love is. Here the symbolism is situated within something far less fragile: the law. One thinks of the law being something one cannot break without punishment and so the symbolism is apt.
Fantasy
Vanity is not introduced in the fourth stanza; it kicks off the motif of comparison. Love becomes a metaphor in the first symbolic answer to the query posed at the start. What is love? “A vain thought” shaped in the mind as part of fantasy.
