Nostalgia as a parasite:

It is now commonly understood that nostalgia can also be a prosocial emotion in the sense that it helps us recognise what is universal to all of us, such as a lost childhood or a longing to be part of romanticised versions of the past
…
The 17th-century medical-scientific literature possessed a weirdly inhuman and morbid philosophy of the effects of nostalgia, their diagnoses of homesickness resembling a grim tale of the fantastique.

Something like nostalgia being a fuel for an overactive imagination. Interesting read.