Why do you like music? I think our brain's urge for survival is the reason.
What is there in music that attracts everyone? And what is special about some songs that everyone everywhere gets hooked to them?
One thing that every song has is a rhythm; some amount of predictability. Human brain likes this... So, one ingredient to a good song is some amount of predictability.
But every song (except in very rare cases) has a rhythm. What is it that makes a 'great' song? One thing I noticed was that, in a good song, there were repeated, subtle changes to this basic rhythm. These changes were scattered throughout the song. The basic rhythm contained a short syllable, but a few times, just quite a few times, the singer drags it quite long. This kind of altering makes a song more appealing.
Predictable, having a rhythm, but having subtle variations.. Where do these strange rules for good songs arise from?
After a few days of 'research' into this, I now have a theory of why it is so. I could call it 'Music as an Evolutionary Survivalistic Aid - an Information Theoretic analysis'. But I'd rather call it the 'better than ya, buddy' theory here.
When we hear a song, our brain tries to store it. If there is a rhythm and predictability in the song, it (probably) makes this easy for the brain. So, that gives us the first rule.
The second rule comes in because of our brain's desire to prove itself better than others.
Everything in our body is optimised for survival. When you demonstrate in public that you are capable of something non-trivial, you increase the chances that the opposite sex finds you attractive; and, in the end, that your gene survives in the form of your kids. Whether we are consciously trying to get this attraction or not, our brain continuously does it. It is something wired in to our very brains, in order to assist in survival.
Now, the second rule tells that there are near-random variations in the song's basic rhythm. This means that storing the song is non-trivial - you need to remember exactly where variations happened and what the variations were. So, as long as the variations don't get too difficult to remember, our brain likes these kind of songs. It thinks(?) that this will help us in our evolution. It's as if 'proving' that we can do this will raise our image in public. So, the brain takes extra efforts to remember these songs. Later, when the song is played when you are among friends, brain remembers and retrieves the song. As long as you sing it right, evolutionarily you are doing the right thing, so more of 'happiness chemical' is released in your brain. This, in effect, encourages you to sing the song right and makes it a 'happy' experience. And guess what? You tell everyone that you 'looove' the song. When this becomes a collective experience, the song becomes everyone's favourite.
And in the same way, when the song becomes too popular, some of us (our brains) decide that knowing the song has become a trivial thing, and lose our interest in it. A newer song has a lot more variety and will be a lot more appealing then...
Note: All this is just my own theory of how all this works, obtained by making interpretations of what I saw at work in my own mind. YMMV...