Music that is predictable is boring. Or music you've listened to a lot and have learned to predict. Music that's too random isn't pleasurable to listen to either. Otherwise you could just listen to white noise. Music seems to teeter in the balance between predictable and random. What could explain that?
The theory is that the brain seeks "novelty" and gets pleasure from it. If your brain tries to predict the next note and fails, synapses adjust slightly and it learns. It gets better at predicting its inputs. Learning these new patterns gives you reward, and encourages you to seek novelty like that in the future. Randomness doesn't cause new learning, neither does predictable patterns you already know.