When does a show become nostalgic?

As a child, I always viewed Saturday as the best day of the week, mainly for the fact that all the best cartoons were on at that time. Whether that was Mickey Mouse Clubhouse, Teen Titans, or the original Ben 10, I can bring up these shows in a conversation with anyone my age and they will get this feeling in their stomach. That feeling is known as nostalgia. However, how does one show, such as The Incredibles, evoke more feelings than another one, like The Little Mermaid?
It would be easy to pinpoint nostalgia to a show that is no longer on the air any more. While this can certainly apply to most shows we view as “nostalgic” this tends not to be the true case. One example is the juggernaut that is SpongeBob SquarePants. The show has been running for the better part of two decades, yet I still feel joyful remembering the early seasons.
Another reason that seems to be more likely is a sort of “generation perception.” While our generation may be fond of things like this Danny Phantom or Justice League, earlier generations may have fond memories of Peanuts or Scooby Doo. This is due to the shows that generations likely grew up watching together. It is very likely that kids in the next couple of years will remember Teen Titans Go and Adventure Time just as fondly.
Perhaps the final, and the most important, reason is just our positive feelings when bringing up something from our childhood. In a study conducted by Alan R. Hirsch, nostalgia is classified as a yearning for an idealized past. This in turn tends to soften all the negative feelings experienced and focuses the positive feelings on one specific thing, like a place or a show. This is why when someone brings up a show you enjoyed in the past, you typically get this joyful feeling in your gut.