Nostalgia a sentimental longing or wistful affection for the past. Remember pleasantries we held onto, that in many ways makes things better remembering, then they were original. It's something that can be quite magical and many times gives us clarity or it can give us some form of a epiphany.
Enlightening and also a learning experience. You can look back and learn lessons you never noticed.
However as magical as the past can be, many people are stuck in it. Where it's a escape, Nostalgia starts to trap you in the old, as your over looking things about it, within it. Things that you probably shouldn't overlook.
Where your no longer learning from the past but wishing your were in it, regretting where or what you are now even if this thought is wrong.
As helpful as it can be there does come a times where you need to cut yourself loose from the old so you can experience the new. The old can be both good and bad.