However, the human brain does nothing frivolously, and there has been new evidence recently to suggest that nostalgia serves a useful purpose in the brain. For instance, the times when people are most likely to experience nostalgia are those when they are feeling lonely or uncertain about their futures. And, despite its reputation as a longing feeling, nostalgia is generally a positive experience, in which even negative memories are tinged with a sense of triumph over adversity. Several new studies have pointed out that nostalgia is probably a beneficial mechanism, a sort of natural antidote to depression. It makes us feel loved, wanted and supported by others -- which is why our friends and families tend to figure prominently in our reminisces. It has been discovered that people who tend to feel a lot of nostalgia also tend to have higher self-esteem and to form more secure social bonds. But is it possible that this is simply a chicken-and-egg scenario, wherein people with happier lives tend to more often indulge in remembering the good times? Then again, even people without a generally positive outlook on life can commonly experience, and benefit from, nostalgic memory.
Memories are not concrete data hard-coded into our brains. Every memory we have is colored with many shades of context. We have a tendency to remember things in temporal "clumps," so a memory from a certain period in our lives can call up otherwise unrelated memories from the same time. This could be due to the hypothesis that when we form new brain cells, they start out especially sensitive to emotional stimuli, then mature and become just like any other brain cells. But a bunch of neurons that all mature at the same time will have absorbed similar emotional input from the time they spent being formed together, and could fire one another as adult cells if the same emotional trigger is encountered again years later. It is true that strong emotional triggers, such as smells and music, are the most likely to set off a nostalgic event in the brain. However, this is only one theory, as modern neurological explanations for nostalgia are still in their infancy.
Nostalgia, like all types of memory, is highly associative. An interesting phenomenon when it comes to nostalgia and our experience of time is that we can actually be reminded of the "good old days" by something we never actually encountered while we were living through them. If I've learned to associate the Terminator movies with the 1980s, for example, they can give me a feeling of nostalgia, even though I didn't actually watch them until after the year 2000 (and all of them except the first one were made after 1990). False nostalgia can even be induced to get people to remember fondly something that never occurred or wasn't even that important to them (advertisers are masters at this.) It all hinges on getting the brain to associate something specific with that nebulous "good" period of time we've stored in our memories.
But why do we tend to remember some things in our past as better than they actually were? Well, most of our nostalgic memories are formed when we are young, from childhood to our mid-20s. These are the times we'll be happily reminiscing about for the rest of our lives, even if many of the experiences we had during those years were not positive at all. The fact is, our brains are always modifying old memories to better serve our psychological purposes. This flexibility in our sense of our pasts, as with most things the brain does, is actually an advantage; it can help us to overcome trauma and to desirably modify our future behavior. This can explain why we become so angry when the rose-tinted memory we have is modified by a source outside ourselves. (See an example here. The outrage!) It is an attack not only on our memory, but the sense of happiness and well-being that we've been able to derive from it. Our brains are using these memories as tools to make us feel good about ourselves and our pasts, which can serve to encourage us to make better futures.