The Necessity Principle states the following:
Essentially, if students question "why" something said in class is true, they are following the necessity principle.
Note, this is not the same thing as being motivated to do well.
For example, one may be motivated to learn the material to do well on an exam because a good grade in the course may lead to some social or economic gain. In that instance, one is not following the necessity principle.
By the same token though, if one is motivated to learn why something works, then one is following the necessity principle. The distinction is fine, but crucial.
Those who follow the necessity principle tend to become scientists and thinkers, for they are naturally curious about the subject they are studying.
Here is a link to my honors thesis: Planting the (Intellectual) Need From Which Learning Grows.
Here is a presentation of a talk that I gave on the paper: Planting the (Intellectual) Need From Which Learning Grows Presentation.