system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some smells universally accepted as good smelling, and some universally accepted as bad smelling?

Everyone I know agrees that cinnamon buns smell good and poop smells bad but why?
gpt
Ah, so there's two theories to this:

There are certain smells that are nearly universally considered bad, and they're usually associated with the really, really bad: poop, rotting/decaying things, spoiled food, etc. Some people speculate that certain smells are just hardwired into our brain as "DANGER, AVOID NOW" because the things the smell is associated with is terrible: you don't want to eat rotting things, or be near poop. So smell is a way of telling you something is to be avoided.

Another theory is that smell, and our definition of it as "good" or "bad" is learned. As we grow up, we're exposed to new things (that sometimes smell) and experience new things: smell can be associated with a good or bad experience and be labeled from then on as "good" or "bad" (Like a happy day at a carnival with cinnamon buns mean cinnamon buns are now good).