“Love what art thou?”
The entire poem is structured as a metaphorical answer to this query. That’s important because the query commences each stanza. The succeeding lines of the stanza refer generally to the overall motif of providing an answer and—generally speaking—to the specific metaphor of each individual section.
“Childish, vain
Firm as bubbles made by rain”
These lines can be interpreted in a number of ways. The speaker may be suggesting that love is fragile, like a child or someone consumed by vanity or bubbles. At the same time, most people outgrow childish behavior, few people are consumed by vanity permanently and bubbles do not last forever. So the interpretation can be taken in a more negative fashion or as something more positive; a brief thing of beauty that should be enjoyed while it lasts.
“A sweet flower
Once full blown, dead in an hour.”
Stanza 3 offers a very familiar metaphor for a love gone dead. What begins in bloom with a sweet bouquet ends too quickly and often smells of rot in the aftermath.
“If thy humor change, to lour”
For the sake of clarity, “humor” is used synonymously with lark or whim. “Lour” is an archaic and little-used word describing change in emotional tone marked by a scowl or sullen behavior.
“Yet alas these are not the worst:
Much more of thee may be said.
But thy law I once obeyed,
Therefore say no more at first.”
The final stanza moves slightly away from metaphor to a more abstract sort of symbolism which suggests that the speaker’s melancholic tone and wistful attitude toward trying to describe love is based on the heat of the moment. The moment of recognition when love has been lost can be every bit as fiery as when love is burning brightly, but it is, of course, a fire that is cold to the touch.