You've got that smile
That only heaven can make
I pray to god everyday
That you keep that smile
He’s hoping that he and her will be able to maintain their happiness together so that she never has a reason to frown, or lose that smile.
Also, it could refer to the apocalyptic scene of the video, where the world is falling apart and he could be hoping that if all else fails, she can still come out of the situation in one piece.
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