Everybody saying it's Tiara's fault, and I normally hate the "everything would be solved if only people could talk to one another" tropes, but honestly in this case I think she's right. She doesn't want him to change his behavior toward the girl that so highly loved by the sea fairies, potentially literally killing the sea from her being distressed.
Honestly, I think it's the Prince's fault. Seriously... look how upset he is any time that his fiance so much as mentions Kieth, let alone talks to him meanwhile he thinks nothing about going on expeditions alone with a pretty girl and leaving his fiance behind. Yes, it's his job, but it's also very two-faced of him. "Fine for me and not for thee", as it were.