It isn't as strange as it sounds. When you don't eat for a long time, you're body gets bored of telling you you're hungry and just assumes you are in a situation where you can't physically eat. Once you start eating properly again, the body responds by encouraging you not to stop until you've made up for the deficit. Unless of course you eat the same amount every day, and then you're body will just get used to that and respond accordingly whether hungry or not.
Is the same with fluids. Most people are so dehydrated that they don't realise it, and when they start to drink fluids (which is out of the norm for some people), the body responds by gasping for even more.
Drinking water is a bit like putting oil and water in your car. As you top it up, you get rid of the crappy old and the car is much better for it. If you don't then the car has to continue recycling the old sludge already within. (This is why your urine goes dark as it isn't allowing much fluid to be lost within it). You'll probably find in the early days you need lots of little trips to the little boys room as your body attempts to flush through the valves until they're working properly again. Like a car, the damage that is occuring isn't always apparent until the engine stops. Same for the body.
On that note, I'm going to go and grab a glass of water.
