you know why you re atracted to her? cause she looks like she has great odds of survival, because she s good looking. It means having an offspring with her equals obtaining an offspring who will have great odds of survival / breeding because of the genes inherited from her. Beauty = greater odds of survival, greater potential to seduce, obtain what you need to survive than others, less good looking individuals. Mating with someone who looks good means your genes and hers in your offspring will insure your offspring with her will have higher odds of survival than others. Basically it means better chances of survival for your genes, hence your "desire" to mix them with hers.