You are wrong, the shots do NOT affect happiness from eating at all. They actively negate hunger and also affect how sugar is manipulated by the body. Nothing they do affects dopamine.
Moreover, you have overly simplified an extremely complex issue, in large part to blame the fat people.
Dietary fiber does not totally suppress appetite. Feeling full discourages eating, it does not negate hunger. You can very easily feel hungry even if you also feel full. That is one of the issues that many obese people face.
Moreover, obesity is not merely an addiction. There are multiple other factors, including exercise. People with physical impairments that preclude exercise are much more likely to be obese, and this has nothing to do with addiction. Depression is related to addiction, but is not identical and also plays a part.
Most important appears to be metabolism. There are people that can eat twice as much food as others and still maintain their weight. And there are people that do not lose weight even if they cut their food intake significantly.
One of the ways you can tell addiction is the correlation with other addictions.
Alcoholism, drug addiction, and gambling are all highly correlated. People that have problems with one are extremely likely to have problems with the other two.
Obesity is slightly correlated, but nowhere near the correlation between the three main addictions.
Does addiction play a part? Yes. but the studies all show it is much less than you suggest.