Throughout "A Complaint," the speaker uses an extended metaphor to explore and explain feelings of loss. This metaphor compares the love that the speaker's relationship once offered to a richly flowing fountain, which becomes stagnant and still upon that love's loss. In this way, the metaphor helps readers visualize the "change" that the speaker mentions in line 1—that is, the difference between life during and after this relationship.
In stanza 1, the metaphor presents the beloved's love as a nourishing and life-enriching force, bringing the speaker a "bounty." It presents this love as abundant, unconditional, and freely given: the fountain's "only business was to flow," the speaker says, and the wealth it brought was beyond the speaker's "need."
The meaning of the metaphor shifts slightly, however, as it is developed in stanza 2. The speaker notes that, with the end of the relationship, the fountain is no longer full of "murmuring, sparkling, living" waters, and instead is now a "comfortless and hidden well." Rather than a metaphor for the relationship, then, this fountain-turned-into-a-"hidden"-well is more a metaphor for the speaker's emotional state before and after the end of this relationship.
The metaphor continues to reflect the speaker's emotional state in stanza 3. Though the speaker hasn't lost everything (the speaker still has a "well of love"), the speaker also knows that the waters of love "sleep" and are relatively inactive. It seems that the speaker can't help but continually compare the present, with its lesser love, to the great love that used to be.