According to Osho, nostalgia is a psychological disease—an escape from the living present into memories—arising because we don’t know how to be totally, passionately, intensely here-now. It reflects non-meditativeness and inner emptiness; the mind compensates by clinging to the past (or fantasizing the future). Nostalgia nourishes no one; it's an old-age tendency, a futile substitute for awareness.
Nostalgia happens when we can’t enjoy this moment, so the mind hides in old memories or daydreams that can’t really feed us.