"We'll always have Paris"

Originally "Nostalgia" was considered a mental disorder. That has changed. "What Is Nostalgia Good For? Quite a Bit":

.... 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 does have its painful side — it’s a bittersweet emotion — but the net effect is to make life seem more meaningful and death less frightening. When people speak wistfully of the past, they typically become more optimistic and inspired about the future. ....

Most people report experiencing nostalgia at least once a week, and nearly half experience it three or four times a week. These reported bouts are often touched off by negative events and feelings of loneliness, but people say the “nostalgizing” — researchers distinguish it from reminiscing — helps them feel better. ....

Nostalgic stories aren’t simple exercises in cheeriness, though. The memories aren’t all happy, and even the joys are mixed with a wistful sense of loss. But on the whole, the positive elements greatly outnumber the negative elements....

".... When Humphrey Bogart says, ‘We’ll always have Paris,’ that’s nostalgia for you. We have it, and nobody can take it away from us....” [more]

What Is Nostalgia Good For? Quite a Bit, Research Shows - NYTimes.com