The abstract of this paper is worth reading --
http://www.ncbi.nlm.nih.gov/pubmed/15987666
Here's the first few sentences --
"Most drugs of abuse increase dopamine (DA) in the nucleus accumbens (NAc), and do so every time as a pharmacological response. Palatable food also releases accumbens-shell DA, but in naïve rats the effect can wane during a long meal and disappears with repetition. Under select dietary circumstances, sugar can have effects similar to a drug of abuse."
Dopamine is slightly complicated stuff. The more I learn about it, the more I realize I don't know much. I hesitate to summarize what dopamine does, because it does a lot of things, and interacts in lots of ways with other hormones and biochemistry.
But, hmm, dopamine comes alongside addictive behavior heavily. We know that. And sugar works heavily on dopamine.
I wish I had a more coherent conclusion to this post for you, but I'm at that early intermediate level of understanding where the layman's basic explanation seems wrong, but I haven't figured out and can't articulate a nuanced explanation. Do read the abstract if you're interested in the topic. Comments welcome.