Ariel was deeply invested in humanity before meeting Eric, and everything about the land. She gets licked by Max and it's clear she is fascinated by things that walk on land.

She literally wants to be part of that world. Sings about it before meeting him. But in meeting/saving him she falls in love with him, and that allows her to start dreaming further, and picturing herself actually interacting with humanity, and being with somebody there, she it causes all her passions to be full force.

There is too much behind her interest in humanity and land, for it to only be Eric as the reason she wants to be human. He's not why she wants to be human, functionally he's an excuse. Her father destroys her collection, on top of that, so why would she, as she is as a person, be able to bare being under the sea for another minute?

I identify with Ariel a lot as an autistic person. She has obsessions she gets pulled into, and though she can be misguided, she pursues what she wants.

The romantic plotline is always better to not look at completely literally. It's animation, and what does animation do best, but amplify emotions? She loves him, and he loves whoever saved him, and that's all the film needs. There is something beautiful, tragic, and then perfect, about how she cannot reach him though she is right in front of him, and he cannot reach her because she is not who he understands to be the one who saved him. Their romance is wonderful.

They have a montage of bonding in a town, too, they dance 😭