Hey, when does Cinderella say that? (I like it, though).
I believe that, truly, you see people and hence find them attractive- we walk down the street, go by some hot hunk, and think 'ahoy!'.
Or some hot hunkess.
But the thing is (apart from the fact that I might think this or that man handsome, and you might not, and vice versa), after you meet the hot hunk or hunkess, if you happen to dislike their insides, you'll stop seeing the beauty in them. That's why I think 'it's what's inside that counts'.
So, beauty is in the eye of the beholder not only because your likes and mine are different (although there are certainly some major beautiful people who we will all agree upon, leaving aside the fact that perhaps and even though we might think them beautiful, we don't have a crush on them); but because the beholder, in learning about the person's personality, sees something else (better or worse) than the rest of people see...
I also think it works the other way around ('ugly' people are made beautiful by their beautiful personality). The problem then is you have to get someone to talk to us, first...

That's why we all want to be beautiful.