A thought provoking piece that can be interpreted in several ways.
Whenever you have a strong belief in something that is going to happen it has the potential to become a self fulfilling prophecy because what you believe can taint your behaviour and how you behave to others.
In the case of the characters in this piece the main character believes strongly that the new love interest is too beautiful and popular to stay with him/her and so seeks to extract a promise that the new love won't ever leave them because based on past experiences people always leave him/her.
Entering into a relationship feeling that way would show insecurity to the new love and would possibly manifest itself in ongoing negative and self depreciating attitude and behaviour which would most definitely contribute in pushing the new love away.
Thus leaving the main character to crow loudly about being right all the time (in the last stanza) whilst at the same time being completely oblivious that they had been the unwitting cause of the loss of the love interest thus the self fullfilling prophecy coming to fruition. A cleverly thought out piece. Milly x