The speaker feels overlooked by someone close to them, and the poem is built around that sting. Each stanza opens with the same word, "Favor-itism," split by a hyphen so it reads almost like a diagnosis. Then the speaker talks themselves out of it. They say they're over-analyzing, that the other person would do the same for them, that they're "complex and hard to do things for," that they want for nothing.
The tone is the giveaway. The politeness is sarcastic, and lines like "what a fool you could never be oh so cruel" and "how rude of me" sound like bitterness wearing a smile. The speaker keeps apologizing for feelings that seem completely justified. By the last stanza, "that sense of complacency" is a sharp jab, suggesting the other person has grown comfortable while the speaker quietly absorbs the hurt.
The mood is wounded, resigned, and edged with irony: someone insisting they're fine while showing the opposite.