Us and them. It doesn't matter whether it is money, class, religion, football team rivalry, or simply two families who have always hated each other, stories about individuals who cross the divide have always proved to be rich seam of material in literature. Think of DH Lawrence's Lady Chatterley's Lover, or Shakespeare's Romeo and Juliet. Americans don't really do class, but they certainly do wealth; the rich and the poor, the haves and have nots. It is bad enough to have such a relationship in the first place. But when one protagonist dies and everyone else starts to suspect you, then things really take a turn for the worse.