Here's why:
* Idioms are phrases whose meaning is different from the literal meaning of the individual words. In this case, "head over heels" doesn't actually mean someone's head is physically above their heels. It means they are deeply in love or infatuated.
* Idioms are often figurative and specific to a language or culture. "Head over heels" is a common English idiom.
While it's a colorful and expressive phrase, it's not considered a specific literary device like metaphor, simile, or personification. It's more of a common expression that adds vividness and imagery to writing.