"It is from the same causes and by the same means that every excellence is both produced and destroyed, and similarly every art; for it is from playing the lyre that both good and bad lyre-players are produced. . . if this were not so, there would have been no need of a teacher, but all men would have been born good or bad at their craft."*
So, says Aristotle, if you want to be good at the lyre, or um. . . piano, you can't just goof around. The good and bad pianist both practice the piano (hopefully) but some of them become excellent, and some don't. The excellent people make a habit of it by repeating the right things over and over again.*
*Nichomachean Ethics 1103b10. My edition is the W.D. Ross Translation.
*There's more expicit stuff about repitition in his later sections on excellence. But that's the subject of a future post.