The mood of the poem seems to shift with the speaker's mood. At first, the poem's mood seems to match his (or hers), but then it breaks from the speaker's mood pretty dramatically. First, he has "a heart fit to break"—fearing, it seems, that Porphyria will not come because of the bad weather—and so he describes the wind as "sullen" and "spite[ful]" in its attempts to "vex the lake." His mood is low, and so his morose and irritated descriptions of the weather (the weather that could keep his lover away) establish the sullen mood for the first five lines.
When Porphyria arrives, in line 6, she lights a fire, making the "cheerless grate / Blaze up," so that "all the cottage [becomes] warm." The mood begins to lift with her entrance and as she works to please and pet the speaker (who playfully does not respond to her initial call). She tells the speaker that she loves him, but
. . . she
Too weak, for all her heart's endeavour,
To set its struggling passion free
From pride, and vainer ties...
(The entire section contains 2 answers and 555 words.)