Maybe because she feels trapped in the body of a mermaid and in her world like a trans person?? What do her material possessions and standing as a sea princess matter if she is unhappy and feels nobody understands her? "Wouldn't you think I'm the girl who has everything?" signifies her plight perfectly. Triton destroying her human treasure trove = a parent throwing a fit over discovering his trans son's stash of dresses and all things girly and wrecking it, right in front of his horrified and pleading child. If the movie had been made now, these themes would have been more obvious and slightly more on the nose like "Let It Go".
Anyways, there is one other reason why Ariel is highly underrated and an aspect nobody talks about much:
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Make it TWICE actually!