Twilight seems to be the most popular star-crossed lover story since Romeo and Juliet. For those of you who haven't seen it or read the books, here's a link to the trailer. It's basically about how a human girl named Bella and a vampire boy named Edward who fall in love. Even though it might seem absurd for two people of a different species to fall in love, it actually makes a lot of sense when you look at their relationship regarding the three major principles.
The first principle, proximity, is the physical nearness the two people are to each other in life. When Bella moves to Forks, Washington, she ends up living in the same town and going to the same high school as Edward. She first sees him in their school cafeteria. A lot of the beginning of their relationship takes place in their biology classroom where they share a lab table.
The second principle, similarity, is the extent to which two people have things in common with each other. Now, being that she's a human and he's a vampire, you wouldn't think they'd have a whole lot in common. But you'd be wrong. They're both very mature for their age (even though he's technically way older than 17). They are both quite intelligent and do well in school. Their families play important roles in both of their lives. Also, both Bella and Edward are physically attractive.
The last principle, reciprocity, is the rule of give and take. Even though both Edward and Bella know they are in love with the other, neither one wants to be the one to say it first because they aren't sure how they other person feels about them. It's only when they both admit to their feelings can their relationship really begin to grow. Throughout the series, each of them has to make sacrifices for them to be together, it's not just one of them that is doing all the giving or all of the taking.