Notice that the normal brain has a lot more red stuff highlighted in it — called Dopamine. This chemical is produced in the part of the brain that is associated with reward. When someone experiences a reward — say while eating a really good meal — their Dopamine (red stuff) level spikes. For addicts, the opposite is true: That spike in Dopamine only comes in anticipation of the reward, as opposed to the actual reward itself. Later, once the reward is gotten, the effects are blunted because the brain has been flooded with dopamine as it thought about eating.
https://www.mic.com/articles/88015/what-happens-to-your-brain-on-sugar-explained-by-science