(Disclaimer: Your mileage may vary)
The idea is that we all have internal (sometimes subconscious) conversations with ourselves about why we do things. We have self-justifications, self-rationalizations, what we believe are personal preferences, and what we believe are things that external forces are forcing us to do.
When we do activity X, we may tell ourselves that there are many reasons for doing X, but one of those reasons is the most important - the primary reason we use in our internal conversation to convince ourselves to continue to do activity X. That reason may be because we think it's fun or that reason may be because we are being forced to do it.
When a parent orders a child to eat his vegetables, the idea is that the child now has an internal conversation with himself: "I have to eat this vegetable because I'm am being forced to by Mom and Dad." If this reason becomes the primary reason (ie. the primary motivation in the child's internal conversation) for eating vegetables, then it pushes aside all the other reasons for eating vegetables. This often results in the child not wanting to eat vegetables because they no longer see any positive reasons for doing it, and only see negative reasons for doing it (ie. being forced to by his parents).
Even in the case of being convinced that eating vegetables is "good for you" - if that becomes the primary motivator, then the person may become resigned to the idea that he must eat it merely for health reasons, not because he actually enjoys it. On the other hand, if he becomes convinced that the main reason for not eating something else is because it's "bad for you", then that leaves him open to the idea that there are actually many reasons for wanting to eat the "unhealthy food" and the only reason for avoiding it is for health reasons.
One might say the same of some people who eventually end up hating their jobs. If their internal conversation about why they do what they do is because they are after money or respect from the community, any internal reasons for actually liking the activity itself may fall by the wayside. The result, then, is that if they lose the money or respect from the community, they also lose the desire to continue at the job, even if it's something they had dreamed as a child would be something they'd find fun and interesting.
See also:
Drive: The Surprising Truth About What Motivates Us
Punished by Rewards