I can't help but interpret that thinking about trying to convince the learner to swallow some nasty medicine. But arguing that School Math is a nasty medicine we're trying to force down kids throat is a different post.
For me, ideas about motivation, especially those of intrinsic and extrinsic motivation, live in the world of behaviorist learning theory. A learning theory that western culture knows so well we have a hard time knowing/thinking outside of of it (like fish & water).
The present constructivist theory of knowing and learning, superseding behaviorism, really messes up the idea of motivation. At first, it makes a different definition for learning. It is not a definition that relies on a "what" is to be learned (what overwhelms us as math teachers), but instead focuses on hypothetical models for knowing and defines learning changes to those knowing structures modeled. So "what" is to be learned is recognized as an idea of the teacher, and something they want to "see" replicated in the learner. Now motivation has become more of a problem OF the teacher, not a lacking in the learner.
What I mean by a problem OF the teacher--as opposed to FOR the teacher. Lack of motivation of students to learn math, to do homework, etc. is the "standard" way of thinking about motivation as a problem of the teacher. The teacher wishes to see behaviors replicated in their students. So totally behaviorist, I can't help but wish to suggest a whistle and a bucket of fish strapped to the teachers hip for rewards to the trained dolphins, er, students.
The idea of motivation as a problem OF the teacher is that when a person's experiential reality is not sitting properly, as it should, that person wishes for it to change -- so as the person's knowing of her experiential reality doesn't have to be shifted to account for something that just doesn't fit. The nature of a knowing, autopoetic organism (i.e. a human) is to maintain its own inner nature, its equilibrium. Herein lies its "motivation." This is the case of a teacher, and of the student. Each's motivation is at odds in their structural coupling. The motivation question shifts to first wonder why would the teacher wish to see her own way of knowing replicated in the knower external to herself, a knower she attributes as functioning in ways alike to herself but is a separate entity. It is the case that she wishes to coerce the other to behave a certain way. The motivation to "teach" is hers, and hence her problem. The question that should be asked is why ought this be the motivation OF a teacher?
A more ethical interaction between two cognizing subjects, even when one may have a socially defined role as teacher and student. The teacher ought not seek a change in behavior in the student, rather strive to invoke an inquiry process that she suspects may lead to a way of knowing that creates for the learner a greater viability in the learner's known world.
For me, I consider the individual knower / learner to be fundamentally goal-directed, i.e. motivated. This is a core principle to their remaining viable in the way of experiencing their world. Any nudge I can give this learner toward experiencing some joy by taking up some logic-mathematical disturbance I can create, might be the extent to which I can concern myself with motivation of a learner. As you can see, I am beginning to find myself thinking in circles about this idea -- when it comes to imaging the work of "teaching."
For now, an unresolved issue...
P.S. Lets not lose sight that Cognitive Psychology remains mired in the tar pool remnants of Behaviorist ideas, especially definitions for learning. The theory gives a reality to knowledge, and fails to problematize the observer stance.
Coda: It is likely I ought to have left this discussion of learning and motivation to von Glasersfled himself. One place he writes directly on motivation is here http://www.vonglasersfeld.com/135, in particular beginning on p. 7.