Notable Examples: Splash (1984), The Little Mermaid (1989), Beauty and the Beast (1991), Shrek (2001)
In fairy tales, unless you're Humpty Dumpty, everything winds its way to "happily ever after." It's something we've grown up with—the beast with a heart of gold gets the girl, the ugly duckling becomes a swan.
But when that's applied to reality, we're less inclined to fall for the fantasy. If Shrek were the tale of a real-life bulbous dimwit, he wouldn't be making eyes with the princess, he'd be maxing out his credit cards on porno websites.
But that's the thing about humans hooking up with fairy tale creatures: life lessons overshadow the plausibility of actual relationships. If something doesn't work, like mermaid bits with human bits, it will work out in the end, as long as you're a decent person. That's kinda nice.