system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why are major keys in music associated with a "happy" sound and minor keys associated with a "sad" sound in our brains?
gpt
A large part of it is cultural reinforcement, major and minor are very rough generalisations of what sounds happy or sad.  
It's not fully understood but is thought the different intervals between notes react to the way the brain understands speech. Over our history our brains developed to recognise certain inflections in a voice's pitch as meaning the phrase is happy or sad.  
A way of speaking where the pitch of your voice varies less is considered sad, your voice seems less energetic. In a minor chord the 2nd note is closer to the first than in a major cord. So your brain just sort of conflates the closer together frequencies to mean sadness.  
It's not very accurate, since it's a clash of 2 fairly arbitrary things, there are many sad songs in major keys and happy songs in minor keys, but confirmation bias seems to override those.