My pet theory is that our brain rewards us for interpreting clues correctly, because this is crucial for survival. And patterns make it easy to do this interpretation correctly, therefore triggering the reward system frequently.

But if it is too easy to interpret a pattern correctly, the reward will be lessened, because the challenge you succeeded in was lesser. And it was also crucial to survival to fade out patterns which don’t change, so that e.g. the wind brushing through leaves doesn’t drown out the noises from a predator approaching.

That’s why patterns which don’t change every so often stop triggering the reward system and therefore bore us.