My conclusion: nostalgia is a love basic to the soul and beneficial to mankind.
As we go through our lives, we subconsciously associate memories with locations, foods, books, music, and many other things. We involuntarily throw ourselves into others. In this way, nostalgia is a love. It is just as much a type of love as is "eros," "philia," or "storge."
This love pulls our gaze off of ourselves. Hence, it is a "proto-morality": the most basic hint at selflessness and love for others. An example: driving past a shopping mall while at home, I remembered the good time I had there with my youth group a year and a half ago. For a time, that memory pulled my gaze off of myself and onto the beauty of the friendships in that group. Nostalgia forces us to remember the good, true, and beautiful moments in our lives.
It also provides necessary elements of human life: stability and joy. Our souls need to return to home. If we are unable to do this, we will settle for returning only in our mind, through nostalgia. It enables us to recall previous joys at the mere sight, sound, or smell of a familiar object. God made humans to feel deeply nostalgic. While we ought not to waste too much time reminiscing, we ought to embrace nostalgia when it comes.