Why does concentrated sulfuric acid behave so differently from dilute acid, and what does concentration really mean?

Why does concentrated sulfuric acid behave so differently from dilute acid, and what does concentration really mean?
The shift from dilute to concentrated sulfuric acid is not just a matter of strength but a fundamental change in chemistry. Above roughly 84% concentration, there are too few water molecules to fully ionize the acid, so sulfuric acid begins self-ionizing to form species like H3SO4+ and HSO4-. This creates a medium that is more like a solution of water in sulfuric acid than the reverse. The dehydrating and oxidizing behavior of concentrated acid stems from its ability to protonate itself, generating highly reactive acylium-type cations such as SO2+ that can strip hydroxyl and hydrogen groups from organic matter. This explains why concentrated acid chars sugars and cellulose while dilute acid does not—and why the reaction is violently exothermic.