This is not only hot, but also touching.
I recently read something that this kind of validation can be indicative of paternalistic society, that a woman can only feel beautiful when validated by a man (it was in context of a pop song). While I think there is some validity to this, the letter in the link is a wonderful way to tell your partner how you feel and provide encourage and positive thought. I don’t think he’s trying to tell her, at all, that she is only beautiful because he thinks so. I think he’s telling her she’s beaitufl because he thinks she is beautiful.