I just finished reading Daniel Levitin's book This is Your Brain on Music, an exploration of the neuroscience of music-making and music-listening. The book is filled with interesting case studies, and compelling explanations of what makes certain musical experiences tickling, addictive, or irritating:
"Music theorists have identified a principle called gap fill; in a sequence of tones, if a melody makes a large leap, either up or down, the next note should change direction... In 'Somewhere Over the Rainbow,' the melody begins with one of the largest leaps we've ever experienced in a lifetime of music listening: an octave. This is a strong scehmatic violation, and so the composer rewards and soothes us by bringing the melody back toward home again, but not by too much—he does come down, but only by one scale degree—because he wants to continue to build tension..."
"[In Beethoven's Ninth Symphony ('Ode to Joy')], the main melodic theme is simply the notes of the scale... But Beethoven makes it interesting by violating our expectations. He starts on a strange note and ends on a strange note. He starts on the third degree of the scale (as he did on the 'Pathetique' Sonata), rather than the root, and then goes up in a stepwise fashion, then turns around and comes down again. When he gets to the root—the most stable tone—rather than staying there he comes up again, up to the note we started on, then back down so that we think and we expect he will hit the root again, but he doesn't; he stays right there on re, the second scale degree. The piece needs to resolve to the root, but Beethoven keeps us hanging there, where we least expect to be. He then runs the entire motif again, and only on the second time through does he meet our expectations. But now, that expectation is more interesting because of the ambiguity: We wonder if, like Lucy waiting for Charlie Brown, he will pull the football of resolution away from us at the last minute."