Nostalgia serves a crucial existential function. 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.
What Is Nostalgia Good For? Quite a Bit, Research Shows
http://www.nytimes.com/2013/07/09/science/what-is-nostalgia-good-for-quite-a-bit-research-shows.html?pagewanted=2&ref=science
Nostalgia helps us deal with transitions. The young adults are just moving away from home and or starting their first jobs, so they fall back on memories of family Christmases, pets and friends in school.
~ Erica Hepper