I think the key differentiator is "reinforcement."

Addiction is often defined [in scientific literature] as having two core elements: Reward and reinforcement (i.e. likelihood to seek repeated exposure).

Addiction in general is a sliding scale. With anything offering both reward and reinforcement being a potential candidate for addiction.

We need protein nutritionally but both the reward and reinforcement are lower than sugar. So if you wish to call protein addictive, you may be able to, but I'd still suggest it is further along the sliding scale towards the "less addictive" side.

Plus if we measure addiction purely by its negative impact on society, sugar addiction claims thousands of lives every year. I don't know how many (if any) lives are claimed by protein addiction.

I recently finished reading Allen Carr's book "Good Sugar Bad Sugar: Eat yourself free from sugar and carb addiction". He's written a number of books to overcome many types of addiction and has clinics to help people with drug addiction. In the book he makes a compelling case that sugar is addictive and has similarities to other addictive things such as smoking, drugs, caffeine, etc.

It's really changed my perspective of sugar.

Is added sugar necessary, though?