I used to boggle at the "they look pretty together" thing until I got into watching Merlin vids: I don't watch Merlin, but am familiar with the basic premise and like Arthurian mythology and pretty people in pretty clothing. I find myself wishing people shipped Lancelot and Morgana just because then the two prettiest people would spend more time together in the same vids :D But on the whole, yeah, it's way more about chemistry than looks.