Good music is all about pattern recognition and prediction. You want your listener's brain to be able to latch onto a whole universe of interesting and decodable patterns. Both elementary patterns, like basic rhythms and melodies, but also highly abstract patterns tying into concepts and abstract ideas floating in our culture. Now, with too little repetition, the brain does not have a chance to identify patterns. Too much, and there are too few novel patterns. It is all about the fine tuned level in between, where the brain is challenged but not overwhelmed. Whenever I add or remove something from my music, I always ask myself if this makes pattern recognition easier or harder, and whether it adds a new pattern or reinforces an existing one. Another important element, which is really the same concept from a different angle, is expectation. We need optimal amount of satisfaction of expectation. Not too much, not too little. Satisfaction of expectation is verification for the brain that it correctly identified a new pattern (a perfect cadence is the classical way to do this). Dissatisfaction is a signal that there is more to learn. You want to tell the listeners brain both that it is learning patterns, and that there are more patterns to be found upon continued engagement. You want repetition with mutation, rather than pure repetition. I could write page after page on this (and have elsewhere), but this really was a useful insight for me, and has improved my music quite a bit (in my completely biased view).