Why do we get thirsty' It has to do with a very important ingredient of our body — water. In fact, our body is two-thirds water. We become thirsty when some of that water leaves our body through our breath, perspiration and urine. Thirst is a signal that we need to replace all this fluid. Our body would have a hard time functioning if we didn’t take in lots of water every day. It would probably shrivel up.