Axiom of Rose-Tinted Glasses
Didn't see anything like this in the wiki, so here goes:

The length of time that passes between actual consumption of a piece of media and the present is directly proportional to a fan's nostalgia for said media.

The formula:

media + (this year - year in which media was last consumed) / fan's initial love = fan's current nostalgia

Corollary: The inverse holds true if a fan consumes the same piece of media again after a length of time.

Thusly:

fan's initial love / media + (this year - year in which media was last consumed) = fan's current nostalgia