One of the criticisms of The Little Mermaid is that Ariel falls for Eric without ever even having talked to him. That he is, in effect, a stand-in for her love of humans. Yet, reviewing that birthday party scene, she learns a lot about him without an exchange of words. He is kind to animals, treats his subjects fairly, is not self-absorbed, loves music, and is of equal rank to herself.
Ariel is a princess. She would be expected to marry well, possibly dynastically. Other than the fact that he's human (a plus in her eyes!), in what way is Eric an ineligible choice? Historically, a lot of royal marriages were based on a lot less compatability than what she was able to deduce from that one spying session. Marriage also used to be more of a political/financial union than the modern interpretation. First you built a stable foundation. Then, over time, as you came to know one another, love frequently grew.
I doubt Disney actually intended that subtlety in the film, as they're telling stories about true love and happiness and it all has to fit into a 90-minute movie. But thinking of it that way may make Ariel's sudden infatuation a bit more conducive to long-term marital happiness. Thoughts?
