The speaker compares a falling-out with a friend to a cookie dunked in tea. At first the cookie is a "sweet treat," but left too long in the cup it softens, breaks apart, and gets thrown away. The friendship goes the same way, ruined by "misunderstanding" and "avoiding one another." Even the tea is called "nasty," and the poem lets the word "vague" slip in, as if the speaker can't quite say what went wrong.
The repetition of the cookie image makes the speaker sound stuck, turning the same thought over and over. The ending is where the real feeling comes out: "i hate you / because / i can't hate you." The speaker is angry, but also still attached, and that makes the anger worse. The mood is sad, tangled, and a little bitter. It's a small, homely image used for a painful, confusing kind of heartbreak.