Because he has never smelled someone that he has wanted to taste so badly the whole time he's been a vamp. Her smell is awesome to all other vamp's as well.At first he hates her, then he's fascinated by her because he can't read her mind. Then he's completly in love with her, can't live without her.. literally
There is just something potent about Bella's blood to Edward. It is like with humans some things smell better than others to Edward, Bella just smells better. And although Bella's blood is what first attracts Edward to her they fall in love in the end. Like in Eclipse when he admits his temptation to drink Bella's blood is gone he obviously still loves Bella. The are definitely in love.