The Hans Christian Andersen tale
The witch only demanded the mermaid's voice as payment because...
- She was testing her to see if she REALLY loved the prince as much as she thought she did. The witch didn't want the little mermaid to sacrifice everything over a teenage crush who she may get over eventually, so she tried to scare her out of the idea by pricing the deal as high as possible. In truth the sea witch COULD have concocted the exact same spell through other means of payment, but simply chose not to while CLAIMING that it couldn't be done any other way. Sadly, the little mermaid wanted a soul and the prince's affections enough to go ahead with it, so the witch went ahead with it anyway, possibly feeling obligated to conduct the spell as she claimed she would, or feeling okay with stripping the mermaid of her voice so long as she was truly willing to compromise it for legs and a chance with the prince and for an immortal soul. The wrenching agony that came with every step the little mermaid took with her new legs may have actually been beyond the sea witch's control, similarly to how doctors cannot always help patients with the side affects to medications prescribed to them.
- A little something I got from one of the fanfictions. The witch had the price be the mermaid's voice so she wouldn't tell the humans of the merpeople. Think about it, she was a naive teenager, and probably one with loose lips. The witch didn't want humans to start going after the merpeople, so to both grant the girl's wish and keep them safe, she took the one thing that would've allowed the mermaid to spread the news (assuming she didn't know the written language of the humans).
"The Little Mermaid" and "The Velveteen Rabbit" are essentially the same story.
- Think about it. Both the little mermaid and the velveteen rabbit desire to achieve a state of being that can only be attained by being loved by a human in some way (for the little mermaid, it's gaining an immortal soul and going to Heaven; for the rabbit, it's becoming real), and these prerequisites are explained to them by two sagely, wizened older characters in their lives (for the mermaid, it's her grandmother, for the rabbit, it's the Skin Horse). Eventually, both characters are able to attract the attention of the object of their affections (the prince and the little boy); however, it doesn't last, as the prince marries the princess he assumed saved his life, and the boy contracts scarlet fever and receives a higher quality stuffed rabbit as a replacement for the velveteen rabbit, who is to be burned along with all his other toys and is quickly forgotten by the boy. Because of this, both characters are overshadowed by someone else more appealing to the ones they love, and are doomed because of it. However, both characters are redeemed through the love that they either felt for others or that others felt for them, and are able to ascend to the respective higher planes of existence they'd both dreamed about; the velveteen rabbit becomes real, and the little mermaid is resurrected as a Daughter of the Air until she is able to earn a soul and go to Heaven.