system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we have nostalgia and how does it work?
gpt
Its more or less a glitch. An incident that made you happy then will still make you happy now. 

You ate pizza with your friends. (At that time it just made u happy cuz pizza n friends)
Now it makes you happy because you had no worries, no debt, no job. Just friends, pizza and pokemon.