This is true beyond members of the opposite sex as well. We associate certain places with beauty, or an event, or art, or what have you. What makes these things stand out, what makes them beautiful, is the effect they have on your mind. Beauty is the instant of awe, of the joy that washes over your body whenever you look at/hear/touch/taste/feel said beauty. In that respect, beauty is defined by how we feel, and by our reactions of the world around us. Beauty is exceptional and memorable, ugliness is forgettable and dull. The mind is trained, for it's own good, to disregard ugliness and concentrate on beauty.

Beauty is so desirable because of the way it makes us feel. On a chemical level, I suppose (and I'm no expert here) that beauty triggers the release of endorphins. Emotionally (this I know a little about), beauty inspires feelings of clarity, balance, perception, contentedness, peace, and a general sense of joy.

One more point...Sometimes, when we come across a thing of beauty, we feel like we being clued in or taught something that we don't currently understand, like we seeing a small part of a big picture. Beautiful things often seem to happen spontaneously, and experiencing them makes us feel humble, like we have been blessed with another piece of the puzzle.

Basically, beauty is a very individual quality, not one we can impose external standards on. It is an emotion, indeed, the very font of all emotion.