It's a little sad, but not "extremely sad"- and then gets spun to sound happy again. It's a children's book that many children still read. I am not deeply religious, but you don't need to be religious to believe in fictional existence of souls in a fictional world.

I get the feeling that most redditors only know the synopsis, without having read the actual story. Yes, technically she does die, but tone of what happens in a story is just as important as the actual details. And the tone of the last part is anything but traumatic.

> The sun rose above the waves, and his warm rays fell on the cold foam of the little mermaid, who did not feel as if she were dying. She saw the bright sun, and all around her floated hundreds of transparent beautiful beings; she could see through them the white sails of the ship, and the red clouds in the sky; their speech was melodious, but too ethereal to be heard by mortal ears, as they were also unseen by mortal eyes. The little mermaid perceived that she had a body like theirs, and that she continued to rise higher and higher out of the foam. “Where am I?” asked she, and her voice sounded ethereal, as the voice of those who were with her; no earthly music could imitate it. “Among the daughters of the air,” answered one of them. “A mermaid has not an immortal soul, nor can she obtain one unless she wins the love of a human being.

And then like 3 more paragraphs about them getting an immortal soul.

He behaved badly, but he wasn't hung for his bad behaviour. He was hung by the fox and cat because they are dangerous criminals. And the cricket didn't have a name.

Which is a overly-happy ending because she gains a human soul. It spends like a sentence about her turning to foam, and the whole last paragraph about what she gained from her decision.