Lemme show you something.DANGER- MUST SILENCE said:snip
"That's what makes you beautiful [to me]."
It's the one guy (well, not literally but in the lyrics) telling this one girl that she's beautiful. It's his opinion. It's not some dogmatic lecture. The singer finds this girl beautiful because she isn't aware of it. Whatever the cause of it, he finds her modesty incredibly appealing.
Having heard this song far too many times, nowhere in the lyrics does it state that her beauty, that any woman's beauty, is reliant upon her continued disbelief in her own beauty. Not only does that make little sense when you think about it, but it also implies there are strict standards of beauty which can be measured and defined. And that's just not the case.