Theme one: The unknown.Ariel's desire to join the surface world is rooted both in her love for Eric and her fascination with humanity. Ariel is captivated by the unknown. Triton's fear of humanity, at least on some level, a fear of the unknown. When he finds Ariel's secret collection of surface-world objects, he destroys it.
Theme two: muteness.Ariel spends the middle third of the film completely mute. Even when she reaches the man she loves and the world she longs for, she is isolated by her silence.
The story is a metaphor for puberty.Ariel is a mermaid and therefore, by definition, a maid, that is, a virgin. She becomes fascinated by Prince Eric, and wants to leave her father's house to be with him. Her overprotective father wants to keep her away from Eric, and safe in his own care. Now watch Ariel's transformation: Ursula gives her legs (and what lies between them), but also takes her voice and her ability to breathe underwater, forcing Ariel to swim desperately to the surface, where she breaks through the water in a cloud of spray, almost completely nude, her head thrown back, her hair flying about her head, with her gasping for breath. To put it bluntly, she looks like she's having an orgasm. The entire subtext of the story is Ariel's sexual awakening. And, of course, the danger she finds herself in is that the man she loves, and for whom she has risked everything, may not love her in return, and may come to love another.