In the case of Diet Coke it's definitely not the caffeine, I think there's something else in the aspartame that affects the addictive quality. Soda has a fraction of the caffeine of coffee.
I keep hearing this...but why is diet 'worse'? I've never seen this explained.
I really believe this is true too... I had a friend who was so addicted to Diet Coke that all day long she would open a new one as soon as the old was empty. One morning, she woke up blind in her left eye. After many many tests and scans, she was diagnosed with MS. I'm not sure what led her to do it, but she quit the diet coke cold turkey, and a few days later her sight started to return. I'm not saying that the two are related at all... but there are starting to be some studies showing associations between aspartame and MS.
Oddly enough, the same week she quit diet coke we were on a
disney cruise. I did the body mass thing at the spa because I'd been having a lot of trouble losing about 15 lbs. I'd been exercising like crazy and eating about 1500 cal/day, and not losing weight. I was also drinking 3-4 diet cokes per day. Again, not saying there is any relation, but when I told him I was drinking diet soda he asked me why... and I, of course, said because it has no calories. He gently reminded me that soda (any soda) is just a bunch of chemicals. He also said (and I"ll NEVER forget this) "No one ever got skinny from drinking diet coke". That's probably not 100% true. I had lost over 40 lbs about 8 years prior when I changed my eating habits -- which also included switching coke to diet coke... Anyway, right then and there I quit drinking diet coke. Done.
I've never looked back. I'll still have the occasional (like 1 per month max!) Coke... And dang if this baby doesn't make me crave soda, so I've been drinking a couple of soda's (sprite or caffeine free root beer) per week... but mostly I drink fizzy water (we call it red neck perrier because we buy the generic store brand!

) or plain water w/ lemon when we go out.
Good luck to all you quitters! I really do believe something about soda is addictive.