it's because everyone dreams of this kind of passion, even with the obsessiveness of Edward, they dream of this fire and this passion, so while reading it they experienced this fire and passion. This link between these two characters, and how they can't live without each other, and how they are tied together by their souls. What person doesn't want that, to find their soul mate in life. And for most real life is dull and boring, and going into the life of twilight, makes their life more interesting for that duration that they are reading. I don't know about anyone else, but I was totally connected to the characters in the books, I cried when someone cried, got mad, when someone got mad, felt loved when the love needed to be felt, and felt happy when it was a time to be happy, and so forth.