How can it be a happy ending story?? Where is the human princess? Things won't be so easy. The prince will choose a princess, not a mermaid. And where is her pursuit for the real soul? Anyway, true love conquers all.
In the end, the only thing of any interest is the paths people take. The tragic part is that even when they know where they're going and who they are. Everything is still a mystery. And that mystery, forever unsolved, is life.