The notion of a human falling in love with a bug-eyed amphibian is so preposterous that the Brothers Grimm used it to test the character of a princess. That fairy tale continues to inspire jokes, memes and endless metaphors (even though in the original version the princess threw the frog against a wall, which triggered his transformation into a prince).
Cranky princesses aside, there is a long, rich history of human-frog love. The amphibians generally mean us no harm, they come in an array of gorgeous hues and that bug-eyed baby face can be ugly cute. They even sing.