Why do we feel nostalgic?

According to Alan R. Hirsch in his report, “Nostalgia: A Neuropsychiatric Understanding,” nostalgia is a yearning for an idealized past -- “a longing for a sanitized impression of the past, what in psychoanalysis is referred to as a screen memory -- not a true recreation of the past, but rather a combination of many different memories, all integrated together, and in the process all negative emotions filtered out.”