As we get older, by definition, our minds starts thinking more about the future. The more i start living in the future, the more my happiness gets destroyed in the present. And so as you become nestalgic, it can recall positive emotions. As a four year old or five year old, their concept of time is not very well developed. Even an eight year old may know what a year is, but they don't think in terms of how is this person going to think about me one year from.