Think back to a time that was enjoyable (at least a bit). A time that you miss.
Do you miss the actual thing, or the time in your life, or the person? Or do you miss the way you felt about it, or them?
It might be that you are nostalgic about the emotion you felt (which, more often than not, transfers and weakens), and not the actual event. Because if the actual thing was still present it’s possible that your feeling for it, or towards the person, may not only be completely different but also completely gone.