The June 2009 Atlantic has a long piece by Joshua Wolf Shenk, "What Makes Us Happy?" Shenk looks at the Grant Study, a longitudinal study (begun in 1937) of 268 Harvard men, and talks with George Vaillant, professor at Harvard Medical School and psychiatrist. Vaillant is the longtime director of the study, associated with it for more than forty years.
I find the following passage especially resonant. Shenk is recounting Vaillant's explanation to a group of graduate students of why "positive emotions make us more vulnerable than negative ones":
One reason is that they're future-oriented. Fear and sadness have immediate payoffs — protecting us from attack or attracting resources at times of distress. Gratitude and joy, over time, will yield better health and deeper connections — but in the short term actually put us at risk. That's because, while negative emotions tend to be insulating, positive emotions expose us to the common elements of rejection and heartbreak.
To illustrate his point, he told a story about one of his "prize" Grant Study men, a doctor and well-loved husband. "On his 70th birthday," Vaillant said, "when he retired from the faculty of medicine, his wife got hold of his patient list and secretly wrote to many of his longest-running patients, 'Would you write a letter of appreciation?' And back came 100 single-spaced, desperately loving letters — often with pictures attached. And she put them in a lovely presentation box covered with Thai silk, and gave it to him." Eight years later, Vaillant interviewed the man, who proudly pulled the box down from his shelf. "George, I don't know what you're going to make of this," the man said, as he began to cry, "but I've never read it." "It's very hard," Vaillant said, "for most of us to tolerate being loved."