Most of us have probably heard that we have to practice music every day. This is because the skillset is developed over time, and the muscle control involved in it needs accurate repetition in order to be second nature.
I would also like to posit that there is another reason, and it relates to why it sometimes feels as if we are getting worse instead of better.
Learning music is a mental and physical process. We are mostly familiar with the physical experiences of singing scales or practicing an instrument. What I find is too frequently left out of the conversation is our change in perspective as we progress.
Our mental and aural acuity increases as does our physical practice. How we listen to ourselves, and music in general, changes. What before sounded just fine, starts to sound out of tune, or strained. Ends of phrases that before didn't get much thought, start to get scrutinized for the style of their release.
Like so many things in life, new awareness seems to come all at once, though it's really the cumulative effect of our daily efforts. So while we may practice merrily along for a few weeks at a time, there then comes that one week where things just don't sound right for no perceivable reason.
There's an old saying that goes something to the effect of, 'True mastery is being able to hear something that is just outside your capabilities to produce it.' In other words, as we get better, we hear and imagine music in even greater detail and intricacy, and so on, and so on.