Alright...let's lay out the assumptions of this argument first...

1.  People learn more efficiently and are more successful doing things they WANT to do rather then requirements

2.  The world needs all kinds of people..."essential" knowledge isn't essential for everyone.

     How can the application of these assumptions to a teaching situation work to create a desire for knowledge in the student?  In the next paragraph or so, I would like to demonstrate how presenting information with the above statements in mind increases students' desire to learn. 

     Teachers in all fields are frequently confronted with the question, "Why do we need to know this?"  This question should always be approached honestly.  It is a disheartening, but real truth that students can survive without knowing a foreign language, without knowing anything about classical music, without knowing trigonometric functions, etc.  And the "you'll need it for high school, college, the real world, etc." statements often frustrate students.  We all know more than a handful of people that prove you don't need to know much to survive in the real world, and many have sat through college general electives that reiterate high school content on a less challenging scale.  An honest approach to the "Why do we need to know this?" question works quite well for the college level (what an intriguing reaction when the question is simply answered, "You don't..." and class continues).  But for younger students studying something "good for them" like a musical instrument or calculus, this same level of honesty can be applied in a different form.  First, the everyday demeanor of the instructor can have an interesting effect on students' perception of a topic.  Watch how the curiosity of a student increases when, instead of presenting your subject as critical and indispensable, you simply say, "Because if you study this you'll know more than many other people," or "just to prove you can do it," or "No one needs to know anything, but a lot of people are impressed by those with such and such skill."  Yes, it is important to praise the mental, social, psychological and other benefits of a given subject.  But if that doesn't work on a student, it is amazing how refreshing an honest response like, "You don't need to know this, but do you want to?" can be for both parties involved.