Chemically, adding CO2 to water creates carbonic acid, which is tasted by sour-sensing taste cells. But if you were an alien studying human taste habits (for the coming food-based invasion?), you might expect carbonated water to be downright repellent: the carbon dioxide in the fizzy drinks triggers pain receptors.
One of the most common is a lack of stomach acid. Basically if your body does not produce enough strong stomach acid, you may start to crave sour foods like lemons because they have high amounts of tartaric and citric acids. Another deficiency that could cause cravings towards lemons is pica eating disorder.