Lisa, an elementary school teacher from Ambler, Pa., came home from work one day and said to her husband, “Honey, guess what? I landed that summer teaching position I wanted!” “Wow, congratulations!” he replied. “I know how hard you worked to get that job. I am so happy for you! You must be really excited.” The way Lisa's husband reacted to her good news was also good news for their marriage, which, more than 20 years later, is still going strong; such positive responses turn out to be vital to the longevity of a relationship. Numerous studies show that intimate relationships, such as marriages, are the single most important source of life satisfaction. Although most couples enter these relationships with the best of intentions, many break up or stay together but languish. Yet some do stay happily married and thrive. What is their secret?