We are just like Pavlov's dogs, in that thinking about a treat like eating chocolate enhances our desire for it and motivation to get it. After we have eaten some, our desire wanes, or habituates. Morewedge et al. make the fascinating observation that imagining the repetitive comsumption of the treat reduces the amount we actually eat:
The consumption of a food typically leads to a decrease in its subsequent intake through habituation—a decrease in one’s responsiveness to the food and motivation to obtain it. We demonstrated that habituation to a food item can occur even when its consumption is merely imagined. Five experiments showed that people who repeatedly imagined eating a food (such as cheese) many times subsequently consumed less of the imagined food than did people who repeatedly imagined eating that food fewer times, imagined eating a different food (such as candy), or did not imagine eating a food. They did so because they desired to eat it less, not because they considered it less palatable. These results suggest that mental representation alone can engender habituation to a stimulus.