Louis knows about beauty; the combination of qualities that pleases the aesthetic senses. He creates that combination every day in the garments he designs while studying fashion at uni. The cut of the design, the color of the fabric, the intricacy of the stitching; it all comes together to create something beautiful. When the science student with the long legs and dimpled smile agrees to model for him, Louis decides he's found beauty personified. Harry just thinks Louis needs someone to show him how beautiful he is.
(please keep in mind that this has not been edited as I am just posting it over here for the people who asked for it)