It was the way he started to woo a princess as if she were some kind of dairymaid, who could just marry at whim -- and whether she repented at leisure or not, at least she didn't cause an international incident. Because of course a marriage of a princess, then heir to the throne, was a matter of state. . . .
Except that to pull that off in a fairy tale retelling, you have to establish a degree of non-fairy-tale realism. Princes go around marrying goose girls and serving girls all the time -- Cinderella is odd among Cinderella variants in that a merchant's daughter was a pretty high birth for that tale -- and princesses to huntsmen and gardener's boys. Yes, there are a fair number where it's a prince or princess in reduced circumstances, but many where it's not -- and of course, a world where a princess's first resort after escaping her father is to get a job as a scullery maid is far on the fairy tale side of fairy-tales vs. realism. And while kings may be annoyed as the lowly birth and stupidity of the hero who fulfilled his conditions, his piling on other conditions always makes him a tyrant and ends badly for him.
In extreme cases, such as "The Grateful Beasts," the hero is chosen as the king, and just happens to marry the late king's daughter as well.
True, you can have fairy tales with arranged marriages ("The Goose Girl") but then, in "Catherine And Her Fate," the king sends his arranged bride back and marries Catherine instead. My speculation that he told his bride that he was now bankrupt -- having, owing to some magic, given Catherine all his wealth -- and that put her off is just my speculation.
Some are easily to put into realism that others -- a Cinderella tale where the prince is required, by his father's will, to marry a woman of his own country, means there will be no princess. Others -- get more challenging. A fairy tale princess whose marriage really is a matter of state takes a fair amount.