Book excerpt: Every troubled couple has had it up to here with self-help books explaining how to make relationships work. This one addresses why they don't-and frankly, why some of them never will. In this unique and illuminating he said/she said guide to avoiding love-gone-wrong, 21 real-life couples spill it to widely-read advice columnist Sherry Amatenstein. What better way to make a relationship last than to learn from the ones that didn't?