From this essay in The Wall Street Journal last Friday:
.... Reflecting nostalgically on the past is a common and healthy experience that helps people find the inspiration and confidence needed to move forward in life, particularly during difficult times. I would go so far as to say that nostalgia is about the future more than the past. ....Clay Routledge, "Remember the Good Old Days? No Need to Feel Ashamed if You Do"
We observed that nostalgia doesn’t cause distress. Instead, distress causes nostalgia. External cues such as running into an old friend, seeing an old photo on Facebook, or hearing music from one’s youth can trigger nostalgia, but when it comes to internal psychological triggers, people tend to experience nostalgia in response to feeling sad, lonely, meaningless and uncertain about where they are in life. ....
They tend to be personally meaningful social memories of experiences such as weddings, holidays, vacations with family or friends, family gatherings and religious rites of passage. They often contain a mixture of feelings, but the positive typically outweighs the negative. Critically, nostalgic narratives tend to follow a redemptive sequence in which feelings such as sadness and loss are overwhelmed by pleasant and even energizing feelings—happiness, love, gratitude and hope. ....
In a series of studies published in the Journal of Personality and Social Psychology in 2011, my research team found that nostalgia makes people feel more connected to those they care about and feel a greater sense of meaning in life. People naturally use nostalgia, perhaps without even realizing it, to maintain meaning. Revisiting cherished memories of times shared with those we hold dear reminds us that life, though sometimes painful and difficult, is also full of experiences that make it worthwhile. ....