According to the internet it started out as "heels over head" in the 14th centruy and then some idiot in 1771 gets it the wrong way round in a poem and now we're all stuck with it. Good shout though, it is daft, i never noticed it before.
i think it came from some movie in the 1920s. yah, if me heels were over my head i'd be upside down. not sure what love would have to do with escher's cluster lizzards either, but you know, doing a forward barral roll, i think thats what its called, head and heels would take turns being over. i think it just means loosing touch with conventionality, in the context of being motivated to do so by affection, or affection as code for libedo.
Why do people say a lot of things? Actually, looking into the origin of some old sayings is kind of interesting. Like "God bless you" after a sneeze, or "Rub the wrong way", etc.
If it were "heels over head" it might just be lust, so I guess if it's Head over Heels it might be real.
along with doing stupid things to impress each other, such as forward rolls: heads heals heads heals heads heals ...
come to think of it, i don't believe i've heard anyone actually speak that phrase for several decades.