This is actually kinda scary once you think about it.

[Chris Brady]

Axis of Awesome - 4 Four Chord Song (with song titles)

NSFW for language.  Like twice...

[Oniya]

It's easy enough to explain.  Rock derives originally from the blues, and one of the classic blues riffs is called the '12-bar' blues - it's 4 bars of the root chord, 2 bars of the major 4, 2 bars back at the root, 1 bar of the major 5, 1 of the major 4, and then 2 more at the root.  You can hear it in the old (Adam West) Batman theme really well. 

Many of those 'learn to play guitar' books start out with teaching the 12 bar.  Throw in riffs and bridges, and you have a pretty decent foundation for music that can fit words, just like learning how to write a sonnet or a limerick.

[Chris Brady]

I suppose that's fair, but...

Pachelbel Rant

[Shjade]

Scary? Not really. A fair number of artforms follow specific patterns that are popular and familiar. Portraits. Landscapes. Iambic pentameter. Specific chords for upbeat songs, other chords for dirges. Character and plot archetypes. So on.

[Chris Brady]

I figure...  The truth is that in the future some scientifically minded music lover is going to build a time machine and go back to find out who wrote Pachebel's Cannon in D.  They'll have the song sheets in hand, and after wandering for a while, realize that they dropped them somewhere.  After a frantic bout of searching they'll find that someone is playing the song.  And then more people will be playing.  So on and so forth.

Once they make the connection, I figure they'll slink away into the machine, and vanish, never to speak of this again.