Most people like music of some kind, even if they are not musicians themselves. But not everybody likes the same kinds of music. So what is music? Why do we like some kinds of music but not others?
Frankly, I'm not sure I can provide good answers, but I know where to start. I define music in a very broad and basic way: music is any sound or set of sounds intended for aesthetic pleasure. Pretty vague, eh?
So let's be more specific. Sounds are used in a variety of ways and situations. A car horn is honked, a doorbell rings, a computer beeps, or a phone rings. These sounds are not primarily intended to be music, but to indicate something--someone's at the door, someone's calling, the computer has a problem, or you've cut a driver off in traffic. But of course, you've probably heard a fancy doorbell, and who hasn't heard any number of cell phone ringtones? Their musical aspect is secondary, though, to their primary purpose.
So when a garage band gets together, or an orchestra performs, the primary purpose is for the listening pleasure of their audience. Whether or not the band is good is a secondary question, the intent is there. Even thirty seconds of silence could be considered "music" if a composer intends it for aesthetic pleasure (see John Cage's 4'33" for example).
While we commonly listen to rock, country, jazz, classical, showtunes, etc, with clearly defined instruments, parts, and musical construction (melody, harmony, rhythm, etc.), usually based on the 12-tone system, much more variety is possible in music. While I'm very fond of artists like Genesis, Steely Dan, and Christopher Cross, more experimental artists like Tangerine Dream and Ned Lagin have created music that is undeniably different from what most people are used to, but which must be considered music, nonetheless, at least by my definition. Whether you like it or not is a different question.
Which brings me to another point: familiarity. Music in general seems to be pleasing to us because of pattern recognition--this is especially true with pop music, but even complex classical works are based upon a pattern that can be discerned, even if only after several listenings.
Of course, thanks to modern technology like multi-track recording and digital processing, even a modern pop song can be very sonically complex, although not in the same way as a Bach concerto. When the rock band Asia came out with their debut album in 1982, one of its impressive technological features was that it was recorded with 32 tracks (for a 4-man rock band, mind you).
On top of pattern recognition, though, is simple repetition. We often tend to like songs more after we've heard them a few times. Much commercial radioplay was based upon heavy rotation of popular songs, but were they in heavy rotation because they were popular, or were they popular because they were in heavy rotation?
One of my early attempts to understand popular music basically just stated that people want to hear what they are already familiar with, but with some slight difference or novelty to keep it interesting. An interesting idea, but ultimately, one that fails to consider the wide variety of musical backgrounds that people come from.