It may come up when one feels that there is something missing. Authentic nostalgia comes from essential value and is accompanied by a longing for something that is beyond (like in Rumi’s poems) and accompanied by a desire to re-access this. On the other hand, the common form of nostalgia is expressed by dreaming of a better (or worse) past instead of welcoming a present necessary suffering. It is a buffer.Often the good (or the bad) of the past is exaggerated and doesn’t accurately correspond to what really happened.