[adrianmonk]

I actually think it might be all of at least three things:

1. Too much sugar has direct effects on the body, even for people at a totally healthy weight. Possibly including changing your appetite.

2. What you said: unhealthy foods very often contain sugar; therefore, foods high in sugar are more likely to be unhealthy in other ways too.

3. Sugar has calories but doesn't make you feel full. Too much sugar works against controlling your weight because people want a minimum amount of satiation.

The first point may seem wrong if you tend to think of the body as simply using sugar as fuel so that it only "pays attention" to the energy content. One thing that convinced me it's probably more complicated than that is a study (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5513783/) showing that if endurance athletes merely swish a sugary beverage in their mouth and then spit it out, it still improves their athletic performance. It can't be merely sweet; artificial sweeteners didn't have the same effect.