First of all, a distinction should be made between “playing” music and “practicing” music. Of course, all practice is playing; however, not all playing is practice. Most musicians and teachers would say that playing is simply the act of running through music (usually music that is already well-rehearsed), whereas practicing is something different – something more. Essentially the difference lies in the concept of identifying areas of the activity that could be considered to be weaknesses, and working to improve them, normally through a regimen that involves a certain amount of physical and mental repetition. In short, practice is the act of trying to improve. So what makes a person want to improve? Much like finding motivation to begin playing an instrument and continue playing it, there are usually certain reasons why people practice. I've found that these reasons usually fall into one of three categories for most amateur musicians (excluding professionals whose livelihood depends on practice and are motivated as such)...
1) External incentives or requirements, which could include parental requirements (sometimes involving rewards/punishments) for children, expectations of a teacher or another person, preparation for an upcoming performance, thoughts (usually in adult students) about the monetary and time investments that go into lessons, etc.
2) Internal incentives or self-requirements, which could include personal goals, the desire to play at a certain level or play a certain piece effectively, the desire to better one's self in general, etc.
3) Spiritual reasons, which could include the desire to bring glory to the God who created you and sustains you, or any kind of accountability to a “higher power.”
Although these can all be valid (especially # 3, which is my own personal motivation to do anything well), there is a reason to practice that is often overlooked: to increase the enjoyment of the activity. The sometimes-hidden fact of the matter is that the the better you play, the more enjoyable the activity is. In short, success cultivates increased enjoyment!
As I stated in one of my previous posts, there is a circular pattern of principles that occurs in practice of which every student should always be mindful: increased practice leads to increased success, increased success leads to increased enjoyment, increased enjoyment leads to a increased practice...increased practice leads to increased success, etc.