From the Atlantic's Happiness article

"Last October, I watched him give a lecture to Seligman’s graduate students on the power of positive emotions—awe, love, compassion, gratitude, forgiveness, joy, hope, and trust (or faith). “The happiness books say, ‘Try happiness. You’ll like it a lot more than misery’—which is perfectly true,” he told them. But why, he asked, do people tell psychologists they’d cross the street to avoid someone who had given them a compliment the previous day?

In fact, Vaillant went on, 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.”