In The Little Mermaid, the titular mermaid wants to gain an immortal soul like a human. A mermaid can gain a soul if they fall for a human. The mermaid just happened to fall for a prince. However, does the mermaid actually love her prince or is she using him to quell her fear of the Cessation of Existence?
It's possible to read the princess who marries the prince as quite manipulative. The prince mistakenly thinks she's the one who rescued him from the shipwreck, and it's implied he fell in love with her for this reason. Did she just carry on letting him believe she did that? It's not surprising the Disney version made it so that this character is really the sea witch hypnotising the prince.
Anvilicious: Andersen had the mermaid transfigured into a daughter of the air, who after three hundred years of good deeds can go to heaven. The moral adds that every child listening to the story can make her labors shorter or longer depending on their own deeds. P.L. Travers snapped at Andersen in one of her folklore essays: "'But a year taken off when a child behaves and a tear shed and a day added whenever a child is naughty? Andersen, this is blackmail. And the children know it and say nothing. There's magnanimity for you."
Applicability: This story is open to quite a few allegorical interpretations.
The undersea world of the merfolk can be seen to represent the pagan world - the first stories of merfolk are from pagan Greek and Assyrian folklore, after all while the human world represents Christianity with its promise of an immortal soul. Compared to the charming, carefree yet soulless "pagan" world of the sea, the humans' Christian world requires suffering and sacrifices, but with the ultimate reward of heaven.
This can also be viewed as a story about growing up. In this interpretation, the merfolk's world represents childhood, while the human world represents adulthood. Like most 15-year-olds, the mermaid is eager to grow up and experience new freedom and adventures, especially romantic love. But when she actually enters the human/adult world, she faces tragic loss of innocence, subjected to pain and hardships she never expected and never able to go back to the safe, happy world of her childhood. In this light, the story can be seen as a warning to children and teenagers not to go chasing after adulthood too soon and particularly to young girls not to throw away their innocence too soon for a man.
Yet another interpretation is about social class relations. In this view, the mermaid represents a person of (literally) lower birth who longs to join the elite world represented by the prince and other humans, but who is doomed to be snubbed by them no matter how much she deserves their esteem. The story can be seen to represent the working class-born Andersen's "fish out of water" feelings among the elite.
Another interpretation based in Andersen's own life is that the story is an early example of Have You Tried Not Being a Monster? The mermaid has feelings for someone she's not allowed to, literally can't speak of her feelings (e.g. "the love that dares not speak its name"), and the prince ultimately rejects her to be with someone more "conventional," his fiancée princess. Andersen himself was bisexual, and is thought to have written The Little Mermaid out of a similar experience in his life: he fell in love with a man he could never have, both because the man was already engaged and because back then, homosexual relationships were a taboo.
Most of these interpretations can also apply to the Disney film, but with a more optimistic spin. The Disney version also has some interesting possible interpretations all its own: see below.
Fanfic Fuel: Just what did the other Daughters of the Air do to earn their provisional afterlife? Were some of them mermaids too? Other creatures? Animals?
Fanon Discontinuity: Some readers and scholars would have preferred for the story to end tragically with the mermaid being changed into foam and dying, rather than the slightly cloying Bittersweet Ending of the mermaid being changed into a daughter of the air with a chance at Heaven.
It Was His Sled: Possibly ironically due to Disney pulling Not His Sled with their adaptation, everyone knows this story ends with the mermaid dying with unrequited love.
Values Dissonance: Non-religious readers often have trouble with the story's moral of "Even heathens can reach salvation if they suffer enough."
Also, modern readers are not strict Christians like the author and have already trouble with that a mermaid has no soul - what is this "soul" anyway? Consequently, one adaption redefined the whole shit the mermaid went through as a Secret Test of Character - by sparing the prince, she proves she had a "soul" from the start (and lives on in varying state of happy ever after).
Funnily enough, the story itself was inspired by Andersen's feeling of Values Dissonance when he read the novel Undine - which has a similar plot of a mermaid gaining a soul when she marries a human. Andersen was not a fan of the idea that a random human being could be totally responsible for someone's salvation.
Values Resonance: On the other hand, unrequited, selfless love resonates in any era. Also, the fact that it can be read as an allegory for homosexuality resonate with many for whom love is taboo.
The Woobie: Dear God, Hans, how much does this poor girl have to go through? For the crime of being born a fay being rather than an oh-so-lucky human, she has to suffer endless pain, have her tongue cut out, commit suicide, and then wait 300 years for her afterlife? And while happy children will take off a year, every tear shed for a bad one will add a day. The story was published in 1837. She could still be waiting for Heaven.
Cult Classic: While less-known than the Disney version, this film has a devoted fanbase.
Fandom Rivalry: Between fans of this version and fans of the Disney version over which is a better adaptation. Some fans prefer this version for being a closer adaptation of the original story and preserving its Downer Ending and take issue with the Disney film for altering it, while fans of the Disney film prefer it for giving the titular mermaid a happy ending. There are also fans who enjoy both versions.
Hilarious in Hindsight: Thor Bishopric voices Fritz the Dolphin, the best friend of the titular mermaid Marina. Years later, Thor would voice Prince Justin in Adventures of the Little Mermaid, which also features a blonde mermaid named Marina as the titular mermaid.
Moe: Just like her Disney counterpart, Marina is a very adorable mermaid.
The Woobie: Marina gives up everything to become human and suffers constant pain on land, is tormented when the prince chooses the wrong princess over her and suffers suspicion and ridicule from the prince's parents and Jemmy the cat, and can't bring herself to kill him and save her own life. By the end of the film, you'll more than likely feel very sorry for her.
The Disney spin-offs
Badass Decay: Eric and Triton in both the sequel and musical.
Undertow from the sequel. With Melody being a copy of her mother, Morgana and her sting rays being copies of Ursula, Flotsam, and Jetsam, and Tip and Dash being copies of...Timon and Pumbaa, Undertow is the only new character with some originality. Doesn't hurt that he's voiced by Clancy Brown.
Benjamin from the prequel. While the movie doesn't sit too well with the fans most agree that he is its saving grace, being an Ambiguously Gay and yet still very cute manatee.
Hilarious in Hindsight: In the St. Louis Muny production of the musical in 2011, Ariel was played by Patti Murin and Eric was played by John Riddle. Seven years later, they'd respectively appear in the Broadway adaptation ofFrozen, with Murin playing Princess Anna, and Riddle playing Prince Hans, who starts as an Eric-type character before turning out to be villainous.
Triton has gotten this a lot more since The Little Mermaid III: Ariel's Beginning, with many fans now seeing him as an oppressive tyrant who randomly outlaws anything he doesn't like (music, the surface) because, in this film, he outlaws his entire kingdom from singing or playing any music because of his grief for his own dead wife. Of course, the fact that this is a direct-to-video prequel made years after the first film, by a completely different studio, with none of the original creative team from the first movie being involved, doesn't stop fans from forming this opinion.
Ariel gets some of this in regards to her parenting towards Melody in the sequel. While Ariel was indeed wrong to lie to Melody regarding her mermaid heritage, many fans are a little too eager to shame her for how overprotective she was of Melody. This ignored the fact that Ariel did try to keep Melody from making the same mistakes she did. Also that Triton was being paranoid about humans in general when he tried to keep Ariel safe, while Ariel was trying to protect Melody from a definite threat in Morgana. Although, it doesn't help that Ariel doesn't explain to Melody what the "definite threat" is, but simply tells her to stay away from the ocean because "it's dangerous," just like when her father told her to stay away from humans because "they're dangerous."
Bordering with a major case of It Runs in the Family and Adult Fear: Triton may have been paranoid, but his heart was in the right place, and he proved to love her daughter so much to sacrifice everything, including himself, to keep her safe and happy. Exactly what Ariel did: she just needed to be a mother herself to understand how hard being a parent is.
Strawman Has a Point: Triton has even more of a point if you've seen Ariel's Beginning and consider it to be canon. Ariel's mother was killed when human fishermen came to steal mermaid relics and accidentally(?) rammed their boat into her. Even if Triton accepted her passing in the end, that doesn't necessarily mean that he thinks highly of humans. It's no wonder he doesn't like humans—they killed his wife!
King Triton in Ariel's Beginning. The guy lost his wife Athena in a pirate attack, and many years later he still can't listen to music without being reminded of that tragic happening. Through the whole movie he looks so sad and depressed of what he had lost, and his strained relationship with Ariel ain't helping him.
It gets worse: by the first movie, his relationship with Ariel is so damaged that she prefers trying her luck with Ursula than trusting him again, he's visibly distraught and blaming himself for her escape and, when finally after years of bickering manages to patch his relationship with his daughter, he's grimly reminded that Ariel forgave him at her own marriage. When he realizes that he's going to miss her, he also realizes that now Ariel has her own family to care for, and he can never, ever get back the time they spent bickering. And then, in the second movie, we find out that he didn't even get to be a grandfather for his (possibly) first granddaughter until she was 12.
Melody in the sequel after being humiliated at her birthday party.