Humans tend to be visual people (unless they're blind, I suppose), and so in general they base their opinions off of the aesthetics of things. That doesn't make it right, but it happens.
That being said, "beauty is fleeting, but a woman who fears Hashem is to be praised." A woman isn't going to be beautiful forever, and if you base your relationship on looks, it's not going to last. This does not mean you have to marry a woman you find to be ugly, but you shouldn't hold out for some bombshell beauty either. If you happen to end up with a bombshell beauty, then cool beans. If you happen to end up with an average Jane, though, don't be disappointed.