Apparently sugar (real sugar and likely high fructose corn syrup, too) actually causes a release of dopamine (the "feel-good neurotransmitter) in your brain, which gives you a "natural high" of sorts. That's why people love sweets, not just because they taste good, but because they physically make you feel good as well (unless you overdose, of course!).
Articifial sugars do not produce this affect. The caffeine and sweet taste of Diet Soda may produce some happiness in people, but not like a dopamine blast.
That's one reason I prefer Regular Soda; if I am going to drink soda, I want to enjoy it to the fullest!
But to each his own. :)
(the dopamine rush can make sugar addictive, but I am going to choose to ignore that for now!)