From What Makes Us Happy?:
… 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.
Then there is this (emphasis mine):
When Vaillant told me he was going to speak to Seligman’s class, he said his message would be from William Blake: “Joy and woe are woven fine.” Earlier in his career, he would use such occasions to demonstrate, with stories and data, the bright side of pain—how adaptations can allow us to turn dross into gold. Now he articulates the dark side of pleasure and connection—or, at least, the way that our most profound yearnings can arise from our most basic fears.