Nostalgia is a common emotion. We usually think that the past was better than today. We may even have a better financial status today, nonetheless, we feel it it was much better when we were a kid.
Even, I recently read that nostalgia has positive effects on the brain. This reinforces the thought that nostalgia is something hardwired into our brains.
Look at some pictures from the 1970s. What is your immediate emotion? I bet it is a surge of nostalgia.
I am not familiar with the neuroscience of this, but I imagine that this is a built-in feature. And it is logical, too. Even though we may have a better life today, but deep down, our brain knows that we are moving toward the end. Little by little, we are coming closer to the end, and that's what causes the feeling of nostalgia.
Whatever its reason or mechanism, nostalgia is usually a bittersweet emotion.