system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some gases odorless (like helium), while others have strong odors (like ammonia)?
gpt
The exact mechanisms of human olfactory detection aren't too well understood but essentially certain molecules fit better into receptors of a particular shape in your nose.  We have evolved and also learn to notice certain smells and find them attractive or repulsive.

Given that helium is inert, non-toxic and unlikely to appear in any significant concentration anyway, there is little evolutionary drive to have receptors that can pick up hydrogen.

Ammonia, on the other hand, is produced in many decompositional processes, can be found in cat urine in large quantities, etc.  Not really something you'd want to be hanging around if it's there in large concentrations.  Thus, we have a benefit from being able to detect it.