Why do we feel nostalgic?

We put an emotional state within an era, or a specific frame, and choose to idealize that specific time. We deduce that because we remember the feeling of happiness at the park, our childhood must have been better than right now.We even place it in inanimate objects, places and smells. Like with Horcruxes (shout-out to my "Harry Potter" fans), we lock away bits of ourselves into things and beings.