system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do our brains/noses decide what smells good and bad?
gpt
It's a bit of nature vs nurture. There are some smells that we are conditioned negatively to from past experience, for example the perfume your ex, who cheated on you, wore, you might experience negative reactions to it (yeah fuck you Jenny.) 

Typically though, shit smells like shit (err.. doo doo for 5 year olds) because it's biological hazardous to your survival, food smells good for the opposite reason.