Even though we all feel nostalgia, it's not entirely clear what psychological purpose it serves. Is this longing feeling for the past good or bad?

Krystine Batcho, Ph.D., a professor of psychology at LeMoyne College, said the word was coined or invented a long time ago, over 300 years ago, and originally designated homesickness. 

"Semantic drift over the centuries has broadened that to the notion of longing for or missing aspects of a person's personal lived past," Batcho explained.

"Most of the research available today including my research argues that nostalgia serves a number of functions," Batcho said. "The thing that ties them all together is that nostalgia is an emotional experience that unifies."

In other words, it helps to unite our self, our sense of who we are, and our identity over time. This can be valuable because over time we change in incredible ways.

"We're not anywhere near the same as we were when we were three years old, for example," Batcho added. "Nostalgia by motivating us to remember the past in our own life helps to unite us to that authentic self and remind us of who we have been and then compare that to who we feel we are today." This also gives us an image of who we want to be in the future. 

Nostalgia also serves an essential psychological function in that it's a highly social emotion. It connects us to other people.

"In the beginning, when we're very young, it's part of what bonds us to the most important people in our life, our parents, our siblings, our friends. As we go through life, it can broaden out and extend to a wider sphere of the people we interact with. It's a social connectedness phenomenon and nostalgia is in that sense a very healthy pro-social emotion."

In itself, nostalgia is somewhat of a conflict because it is a bitter-sweet emotion. But that also has a hugely positive effect because it helps us to unify what otherwise would be felt or experienced by us as conflicts.

"The bitterness comes from the sense that we know for sure that we can never really regain them, they're gone forever. The irreversibility of time means that we absolutely cannot go back in time so it helps us to deal with the conflict of the bitter longing for what can never be again together with the sweetness of having experienced it and being able to revisit it and relive it again," Batcho said.

According to Batcho's research, nostalgia can be a stabilizing force and comfort us during times of transition.

"It's very difficult to grasp change, because in some sense, at a very deep psychic level, change threatens us," the psychologist explained. "It's a little frightening because we're not 100% sure that we can control it. One of the most important aspects of being a healthy human being is having a sense that you are in control of things. When things start to change, either very substantially, such as major events in a person's life, getting married or getting divorced, getting a new career, going back to school or graduating from school, it's comforting to have a nostalgic feeling for the past that reminds us that although we don't know what the future is going to bring, what we do know is that we know who we have been and who we really are."

Nostalgia brings things back. It stimulates memories of the times when we were accepted and loved and this phenomenon is very comforting. Knowing that at some point in life we didn't have to earn our love can be very powerful. Nostalgia helps us realize that our parents, our siblings, or our friends simply loved us unconditionally. And these thoughts can work miracles on us when we're undergoing turmoil in our personal lives.

Batcho believes nostalgia might even have an element of infectiousness and wishes it was even bigger. To understand what she means by that, think of a group of people who have some shared experience.

"The best example of that is when a group of friends are reminiscing about their antics, the things they did as either teenagers or children or what have you. There it's a bonding experience. Another example of that is we know from current research that sports fans love to do that. They talk about the games that they enjoyed and when their team was victorious. When you have a common bond, it can be very infectious and it's like a domino one memory triggers another."

And the reason why she wishes it was more infectious is that this, let's call it, collective nostalgia can break down when you have a group of people who do not have shared experiences to be nostalgic about. Suddenly if you have one individual wanting to reminisce about their past but no one else in that group remembers those things or lived through those things, you might get things like a generation gap. Someone is talking about the 1950s and a teen says they can't even imagine it.

So when you think about it, r/Nostalgia is doing a beautiful public service. The subreddit is bringing together those who might not have anyone to daydream about the past with.