system
Given the following post, respond as if you're commenting on Reddit:
human
Is there a psychological reason we recognize happy/sad melodies, or is it just that we're used to the context?
gpt
Not so long ago major keys were considered 'happy' and minor scales considered 'sad'. But it has been determined that that is just conditioning.

Also in music there is the idea of tension. Try singing the first 6 notes of Twinkle Twinkle Little Star, leaving of the seventh ("star"). That distinct sense that the melody has been left uncompleted is *tension*, and the seventh note is thus said to "resolve" the tension. Always resolving the tension makes for more positive effects of the listener, so nursery rhymes will reliably always resolve tension.