Nostalgia
In the dictionary, the definition of nostalgia is " a sentimental longing or wistful affection for a period in the past". So nostalgia is a kind of missing something or being homesick. I think it's a good thing to be 'reminded' about good things that have happened, as long as we can remember that our reality is also fine and the time that has passed will never come back, but the good things will still keep happening. If we can manage to keep that perspective I think it's good to feel nostalgic once in a while. On the other hand, it can be very painful, for example, for a person that is far away from home: homesick and alone.I think it's not easy to have a mindset that wrote about above so for most of the time nostalgia is not a good thing.