system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is nostalgia, how does it work, and why do I feel it?

I was recently watching Avatar: The Last Airbender on a whim, and ended up doing a marathon over a couple weeks watching the whole thing. I watched the show all the time as a kid, and loved every second of it. These days, I don't really find much enjoyment out of watching it, since it's aimed towards kids. I did, however, feel a lot of nostalgia, even finding myself crying for just about no reason while watching the show. It got me thinking... why am I even nostalgic for this show? I don't have any memories of it that stick out, and my childhood wasn't abnormal in any way. Everything I've searched on the subject is either too vague or confusing, and so I ask you to explain this like I'm five. What IS nostalgia, how exactly does it work, and why am I feeling it?
gpt
*nostalgia* is the happysad feels you get when something in the present remembers you of something in the past. 

it works with memory. your brain has little boxes full of memories. each box has a time and a date on it. you cannot see inside the boxes but your brain can. 

sometimes what you can see or hear or touch or smell or taste reminds your brain of something that you saw or heard or touched or smelled or tasted in the past. then your brain will go to that memory box and open it and stick its tongue inside to make sure it's just right and then it will drink the contents of the box and then it will vomit those memories all over your face. then you feel happysad.

you feel it because Never Forget.