Sugar isn't strictly bad for us. The body uses sugar for a lot of important things like being the primary source of energy in the body. Too much of anything is bad and we just tend to have too much sugar. When we have too much sugar it throws our body out of balance and can even lead to the body not producing the natural sugars that it needs.

Cravings are the body's way of telling us we need something. When you're dehydrated you crave water because you need it. Sugar is one of the main ways we get energy so the brain sends out reward signals when we ingest sugar because it wants to keep getting high-energy foods. In nature, it's difficult to get energy-dense foods like that so it wants us to keep eating more. The problem is, with modern society we've made it too easy to get processed sugar. Ultimately, it's just too much of a good thing.