It's the fructose. The biochemistry behind it is complicated, but we don't process it as well as other sugars. Large amounts cause mild euphoria, followed by headaches and general malaise.

[She glances again at the soda can in his hand.] It's fine in moderation for most people - a small serving of fresh or dried fruit wouldn't hurt me, although my tolerance isn't what it was when I was younger? But I checked earlier, and most of the carbonated beverages seem to use the concentrated syrup form as a sweetener.