Mabel fashions herself an expert matchmaker, but her pairings are forced and arbitrary. Either way, she doesn't take anything into account, including wishes and preferences of subjects. She sets up Robbie and Tambry, and it predictably fails, because they don't like each other in any way.
So Mabel being Mabel decides to double down and mess with their minds because she can't accept being wrong. Which went about as good as before, because high on love potion kids are annoying and unreasonable.
But it works in the end anyway, because fuck you, that's why.