Is it something that happens to you when
you remember the sticker book you had created when you were in school?
you remember fight caught with the worst kid in class?
you hear a song you had first heard 20 (or may be 15) years ago ?
you smell the same food you had tasted first on a rainy evening?
you reach the same place where you had kissed you girlfriend for the first time 10 years back?
you come across your old photos on a college picnic?
you miss the warmth of your mother love?
you realize you have lost the watch gifted by your father on your 15th birthday?
I dont have an answer but I think nostalgia is a revelation that something that happened in past with you was so good and important that you miss it in your life though you failed to realize its importance at that moment of time.
Incidents happen, we think they were trivial but we miss the point that they have the potential to bring fond memories one day.