The two men met in 1990, on a train from Indianapolis to Salt Lake City. In their early 30s at the time, they struck up a conversation while waiting in line in the snack car, then continued talking for hours. When they arrived in Utah, they exchanged numbers.

Over the next few years, Kevin and Ron and their wives became close. They gathered for birthdays and ballgames, shared meals and confidences, and celebrated the arrival of their firstborns.

Then jobs took them to different cities. Calls and cards trickled to a halt. By the time the pandemic hit, they hadn’t talked in about 25 years. Yet Dr. Masters found himself increasingly thinking about his old friend.

“The idea of reconnecting with someone who knew me when I was just starting out seemed like a powerful way to feel grounded,” says Dr. Masters, 63 years old, a clinical health psychologist in Denver.

Missing old friends? You’re not alone. Pals from our past can give us a sense of stability in turbulent times.

Research shows that psychological distress often causes nostalgia. People tend to experience this sentimental longing for the past when they are feeling sad, lonely, anxious or disconnected, or when life feels meaningless or uncertain.

“Covid represents a big sense of discontinuity in our lives. We’ve lost a sense of who we are,” says Clay Routledge, a psychologist and professor of business at North Dakota State University, who has studied nostalgia for 20 years. “Recalling cherished experiences from our past can remind us who we want to be, who we want to be around, and what we feel is important in life.”

Nostalgia increases positive mood, self-esteem and self-confidence, according to studies conducted by Dr. Routledge and others. It makes us feel more socially connected and optimistic. It helps us feel that life has more meaning. And it’s highly motivating, pushing us to pursue goals, reconnect with people who were once important to us, and make new relationships.

We can become nostalgic about any period in our life. But it’s most common to feel a longing for our adolescence or early adulthood, likely because that’s when we developed our sense of identity and forged our own relationships.

Dr. Routledge says that most people feel nostalgic about social experiences, typically with family or friends. We may long for their support or feel we can trust them. Old friends—especially ones from our youth, who may also know our family—are often the people we believe truly understand us.

One recent week, I received emails from three friends I hadn’t heard from in decades—one I met in my first job, one I worked with at The Wall Street Journal 20 years ago, including in the days after the Sept. 11 attacks, and one from summer camp when I was 15. Each letter improved my mood immediately, and when I wrote back I felt less isolated.

I got a boost of energy from the memories the letters inspired. Thinking about my younger self and the goals I had made me buckle down harder on my current deadline. The experience was so lovely that I sat down and wrote a list of all the friends I miss. Then I made a plan: Each week I’ll reach out to one person on the list.

In reporting this story, I heard from people who have reconnected with best friends from high school, college roommates, former boyfriends and girlfriends. They reached out after finding letters from their friend stuffed in a long-forgotten box, moving closer to their childhood home, or learning of a death or illness.

This summer, Rebecca Brooks, 50, wrote to Kimberly Gilligan, a college friend she hadn’t seen in two decades, after seeing the woman’s Facebook posts on spirituality, her family and her mother, who had died. “I’d been feeling disconnected from friends and was craving a deep connection,” says Ms. Brooks, who owns a public-relations agency and lives in Morristown, N.J. “And I saw that we had a lot in common.”

In July, the friends reconnected over Zoom and have since had two in-person meetings. When they met at her home, Ms. Brooks pulled out her old journals and the women sprawled on her bed and read them. “I was reminded of the moments that built me,” says Ms. Gilligan, 52, who owns a clothing company and lives in Fort Collins, Colo.

Now the friends regularly send texts and videos. And they are planning a third gathering. “There is nothing like a friend from adolescence; they get to know you and your family in a way that no adult friends do,” says Ms. Brooks.

Dr. Masters searched the internet for Ron Grant last fall. He felt a sense of urgency: He and his wife planned to rent a house in the Seattle area for a month and he knew his friend lived in Tacoma. When he found an email address, he wrote and asked if Mr. Grant and his wife wanted to get together. Mr. Grant responded, and they made plans for a visit.

On the appointed day, Dr. Masters was nervous. “Will it be like I remember it?” he wondered. But when he opened the door, Mr. Grant threw open his arms, hollered his name and immediately made an inside joke. The two couples laughed and hugged. “And the party was on,” Dr. Masters says.

For the rest of the weekend visit, the two couples talked—about their families, careers, current events and future plans. Mr. Grant, 61, a life insurance program manager, shared a personal story he hadn’t told many people. And Dr. Masters also shared something intimate. “He told me I was like a brother to him,” says Mr. Grant. “That was really special.”

“It was like a deep grounding in my soul took place, being back in touch with folks we had a history with,” says Dr. Masters.