John Tierney writes about the benefits of reminiscing in What Is Nostalgia Good For? Quite a Bit, Research Shows:

Nostalgia has been shown to counteract loneliness, boredom and anxiety. It makes people more generous to strangers and more tolerant of outsiders. Couples feel closer and look happier when they’re sharing nostalgic memories. On cold days, or in cold rooms, people use nostalgia to literally feel warmer. […]

Nostalgia serves a crucial existential function,” Dr. [Clay Routledge of North Dakota State University] says. “It brings to mind cherished experiences that assure us we are valued people who have meaningful lives. Some of our research shows that people who regularly engage in nostalgia are better at coping with concerns about death.”

Of course, there’s a fine line between reminiscing about great moments, and allowing those memories to make you feel depressed. In the words of Stephen Stills:

Don’t let the past remind us of what we are not now.