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 dissever,
And give herself to me forever.
Porphyria says that she will not be able to stay with the speaker, her lover, due to some vanity on her part (perhaps the narrator is of lower social status and their relationship would cost her social position). We might await a change in mood here because she has just told him, essentially, that their relationship must come to an end. Instead, however, of the speaker becoming depressed, he seems to become happier. The narrator describes "to-night's gay feast" and how he feels "Happy and proud" to know that "Porphyria worshipped [him]." His personal mood elevates as he decides to strangle Porphyria with her hair and, in this way, keep her with him forever. Meanwhile, the mood of the poem becomes completely chilling as we realize that he has murdered her in order to compel her to stay with him. The mood of the poem becomes threatening and unpredictable, as we have no idea what else this speaker is capable of.