If you’re not familiar with the theory on drinking water and thirst, you should research this out–but in sum, when you don’t drink water, your body responds and actually shuts your thirst mechanism down–so you lose your natural thirst. One of the first things people notice when they start drinking more water is that after a couple of weeks, they are really thirsty, this is your natural thirst coming back which has been shut off due to dehydration.
Also, if you have not been a water drinker and suddenly start, you’ll notice you will pee a LOT the first couple weeks. This is because your body isn’t quite assimilating it yet. I’ve heard the analogy of a very dry potted plant. When you pour water in, it all goes out and doesn’t soak into the soil. It’s not until the soil is properly hydrated that the soil can retain the water at normal levels. Apparently this is similar to our bodies–and you do notice this pee phenomenon when you start on the water, so it seems to have some truth to it.