i personally believe, like Robert, that it just has alot to do with two elements of the story. Thats death and sex. And because these two are already so closely related not just in this book but in life it kind lights a fire in a females or maybe even a homosexual male to hear someone say 'i love you so much....i want to kill you' now listening to that you probably think thats insane but if you look at it as if the situation was an adrenalin enhancement i think you could understand.....one perfect example is that in the second book 'New Moon' Bella went to port angeles with jessica and she thought that she saw the guys that were going to hurt her in the first trip. Instead of running in fear she walked toward them with passion in her eyes because she could hear her lovers voice despite the fact that he was so outraged by her actions. she found that danger and complete recklessness was the key to unlocking that part of her subconcious to hear and see her mate. And being human we are all a bit irrational at times and i think that is what really starts those butterflys in our stomach.....knowing something is so dangerous and so wrong that it just has to be right.