10. They do not fall in love. They are love.
9. The male magician enchants the mermaid by persuading her that he is a merman and he is here to offer a higher and greater love.
8. She drops by out of curiosity to see if human beings have learned anything more about love since she was last here twenty thousand years ago.
7. The mage makes a deal with fate to possess a mermaid against her will in exchange for sinking his continent beneath the waves a few decades later.
6. Mermaids love everyone, even jerks.
5. The mermaid realizes that human beings have this odd rule about exclusive relationships so she tries to fit in by acting and pretending she is a one-man kind of girl.
4.The fisherman catches the mermaid in his nets. But the daughter believes the mermaid is a selkie. The mermaid goes along with the fiction in order to reduce their anxiety.
3. In a last ditch attempt to save mankind, the mermaid incarnates in human form and looks around for someone to love in order to better communicate her message.
2. The mermaid is not in love with a real man. He is actually a merman in a man's body.
1. She drops by to teach men that love is everywhere and in everything. She fails. So she tries to teach men the difference between touching a woman's body and a mermaid's body. Some get it.