Two and a half years ago, Virginia Ng had eating habits a dietitian would be proud of. The 25-year-old public relations professional from Seattle cooked up healthful veggie stir-fries every night and rarely ate dessert. She also exercised regularly, mixing up her routine with aerobics, jogging and weight training. She was the picture of good health. That was then. These days, Ng isn't nearly as careful with her diet and physical fitness; not surprisingly, she is 25 pounds heavier. What's her explanation for the healthy-living downturn? Finding a boyfriend. "I seem to get into a mental and physical rut when...