Every time you eat sugar or starch, the bacteria in plaque produce acid that dissolves minerals from the enamel. Saliva repairs some of the damage, but frequent snacking tips the balance toward loss, and a soft spot slowly becomes a hole.
The key insight is that how often you eat sugar matters more than how much. Grouping sweets with meals, rinsing with water, and letting teeth recover between meals dramatically lowers your cavity risk, and fluoride toothpaste helps rebuild early damage.