It's something I almost can't put a finger on... this song affects me on something like a subconscious level.
The song is beautiful, that's indisputable. There's a reason it's one of the most famous songs in American film, and in world culture in general. Judy Garland's performance is beyond her years in wistful gravity, with a strain of knowing sadness.
It's that sadness that leads me down a rabbit hole into deeper contemplation.
This song (and Wizard of Oz as a whole) is, to my knowledge, the first example of camp sensibility in entertainment. To explain that, look at the song. At the point that Dorothy Gale sings "Over the Rainbow," (which I guess is the official title) she has just experienced viscerally for the first time how malevolent forces have unnatural power over her life, in the form of Ms. Gulch, who intends to execute her dog Toto out of pure reactionary hate. This is happening out in rural Kansas, seemingly one of the most serene places you can find on planet Earth. If even this place is dominated by the whims and connections of Ms. Gulch, where else can get justice?
It's a dark situation in which she suddenly finds herself, but does Dorothy sing a song of rage and dejection? No: She performs one of the sweetest and most elegant escapist odes ever composed. There's a sadness in it, but it's more in Dorothy's knowledge that the happiness she sings about is not nearly possible, even where she's at. This is, I think, the core of camp - responding to the nihilistic abyss by retreating to a knowing pleasantness, even silliness. That alone makes this scene historic.
I think this song's attitude strikes a major chord with all people, which is another reason they gravitate to it.
Dorothy Gale is essentially powerless throughout the film. Toto attacking Ms. Gulch was beyond her control, as was Ms. Gulch's response. She is powerless to stop the tornado from lifting her into the land of Oz. She is sent by the Glinda down the yellow brick road. She meets her companions by happenstance on the way. She undertakes the mission to defeat the wicked witch only on the Wizard of Oz's orders, then defeats the witch by pure accident as she bumbles ineffectively with a pail of water.
The only measure of control she is allowed in the film is in one action: Escape. Early on, she briefly runs away from home to save her dog, separating her from her family just in time for the tornado to hit, sending her off on a separate trajectory.
If there is one idea that "Over the Rainbow" rests one, it's the idea of peaceful, positive escape. It's not exactly the ideal "manly" American solution traditionally prized in our mythology.
But I do believe it's just as popular - and valid.
Because human beings are feeling people. Positive people. People who would rather be over the rainbow rather than facing down wrath. No healthy person should actually seek to be the fierce, heartless combatant of our militarized legends.
And remember, in the end, even a hero so assertive as Luke Skywalker didn't defeat the Emperor either. Darth Vader was his unintended pail of water.