Ventral tegmental dopamine neurons respond only to UNPREDICTED REWARDS. When the goodie is predictable, those particular dopamine cells stop firing. They's the ones that say "Oh YEAH" to something one discovers as enjoyable and satisfying. Smoking a cigarette fires those cells even when it's not unpredicted, it provokes that Oh YEAH chemistry every single time you light up.
It's the dopamine release in the nucleus accumbens that stengthens the links between the stimulus and the wanting. Wanting and liking are not the same.
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