system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do different songs illicit different emotional responses. Why do some songs make us ‘happy’ and others make us ‘sad’?
gpt
Short answer: Because songwriters have spent a lot of time learning about the elements of music, and use them to trick your brain into feeling a certain way. This only works because you've heard a lot of music before, and learned the "language of music" without even knowing it. 

Long answer: Music is made up of a lot of elements. Let's take a look at a few of them. 

* Lyrics. This is the only element of music that can be truly "sad" or "happy." If you never heard a song, but read its lyrics online, you could probably tell if those lyrics told a happy story or a sad story. For centuries songwriters have paired sad lyrics with certain other elements of music, and now it only seems natural that those elements of music are associated with feeling "sad." The same is true for "happy."

* Tempo. This is how fast a song goes. We find faster songs to be more upbeat and happy, while slow songs are more down and sad. This just seems natural to us, and songwriters have used that. They tend to pair happy songs with fast beats, and sad songs with slow beats. Is it possible that it's just human nature to associate fast with happy and sad with slow? Maybe. But at this point, it's as much cultural as anything. We're so used to hearing those songs in that context and we can't separate the emotion from the tempo. 

* Harmony. This is when two or more pitches are sounding at the same time. In music theory, we can analyze these groups of notes and give them names and categories. These are called chords. Some of these chords are associated with happy emotions, and some are associated with sad emotions. This is almost entirely cultural. In the west, we associate major chords with happy and minor chords with sad. In Persian music, for example, minor is simply neutral, and there are other chords that are used to achieve sadness. It's mostly because slow songs with sad lyrics tend to use a lot of minor chords that we (in the west) think minor means sad. This is something your brain understands even if you don't know anything about music. 

* Melody. This is the part of the song you would hum or whistle. It's the "main" bit of the tune. Songwriters don't just choose notes willy-nilly to make songs. Songwriters use scales, which are a certain group of notes that tend to always work well together. It's sort of like choosing a pallet of colors to work with. Once a songwriter chooses a scale, they will write melodies using notes from that scale. Just like chords can be major or minor, scales can always be largely grouped into major-type scales and minor-type scales. Again, this is cultural. Because sad slow songs tend to use melodies from minor scales, we associate minor with sad. Happy fast songs tend to use major type scales, and we think major means happy. It doesn't have to be that way, but we've heard so much music in our life that our brain is wired to interpret those scales that way. An alien would have no way of knowing what scale meant what emotion. 

* Putting it all together. Skilled songwriters can put these elements (and more) together to make a whole range of emotions. What about a minor song that's also fast? Maybe that feels manic. Or a major song that's slow? Maybe that feels whistful. Or a medium tempo song that's in a major key but emphasize a lot of minor chords? Maybe that's bittersweet. It's amazing how music has the power to go straight to your brain and make you feel a certain way, and it's all just mixing and matching these (relatively) simple elements.