In a series of studies published in 2006 in the Journal of Personality and Social Psychology, subjects were first asked what leads them most often into nostalgia. The most common answer was not scent, or music, or even reunions with old friends. It was bad moods [source: Wildschut et al.].
Negative feelings appear to be major nostalgia triggers, with loneliness topping the charts [source: Routledge]. It makes sense: Nostalgic memories tend to revolve around positive social experiences — good times with friends, laughing at the "kids table" at Thanksgiving dinner — and in a lonely state, calling on those memories, and yearning to relive them, is a natural response [source: Routledge].
But it seems like almost any dark mood can lead to nostalgia. In one component of that 2006 research, subjects read one of three different news stories. One reported on a tsunami disaster, another on the landing of a space probe, and another on the birth of a polar bear in a zoo. After reading, subjects answered questions assessing their current levels of nostalgia. The subjects who read about the tsunami were the most nostalgic of all the groups [source: Sedikides et al.]. When researchers induced existential angst in a group of subjects (by having them read an essay dealing with the "cosmic insignificance of human life"), those subjects nostalgized more than those who'd read essays without any existential angle [source: Routledge].
Any emotional state that might defy a positive outlook — any "psychological threat" — increases the likelihood of nostalgizing [source: Routledge]. This might seem counterproductive: If we're feeling bad in the present, wouldn't emotionally connecting with a happy past just make us feel worse by contrast? Once the wistfulness subsides, it seems we'd ultimately be left with "I was happier in the past, and I can never go back there."
Surprisingly, it seems to have the opposite effect. Nostalgia, contrary to centuries of common knowledge, appears to be a good thing. A really good thing.