Not any one thing. Sometimes it can be the melody. I have found that it doesn't necessarily need to be a bouncy ditty for it to be catchy. A slow ballad like "Danny Boy" has that something to it (of course, I know that one is old and well heard).

Sometimes it is the way the lyrics are married to the music. (There is a term for the music following the meaning of the words, but I can't remember it) I think it is such a marvelous expression to have the music "weep" when the words are crying, if you see what I mean.

Sometimes it is the way the music takes a journey through chord progressions, JS Bach being a master at that! No words, but a driving beat, a walking bass, and a melody that keeps changing until you've heard it forward, backwards, and inside out! And all very pleasing to the ear.

Margarita