A relationship is ending, and the speaker is the one who sees it clearly. The first eight lines repeat the same pattern: the other person says "I love you," then something in their body or voice gives them away. Their eyes are empty, their gaze drifts across the room, their voice falters. The repetition makes the words feel hollow, as if the phrase has been said so often it has stopped meaning anything. The blunt line "but you just didn't" drops the excuses and states the truth plainly.
Then the poem turns. The speaker says "I love you" and meets the other person's eyes, which is the opposite of the earlier avoidance. Both of them know what that means. The final line, "At least one of them was true," is bitter and quietly sad. It suggests the speaker's love was real, while also admitting the other's wasn't. The mood is tired, hurt, and resigned, with very little anger left.