Musicians enjoy performing music. Mathematicians enjoy doing mathematics. Soccer players enjoy playing soccer. Of course, it is possible that this is completely due to a self-selection process—that the only people who would spend years practicing something are those who naturally love to do it—but it is also possible that the practice itself may lead to physiological adaptations that produce more enjoyment and more motivation to do that particular activity. That is nothing but speculation at this point, but it is reasonable speculation.
