I am thirsty, and I think the whole world deserves to know. They deserve to know also, why I feel thirsty, and why they feel thirst. I have also decided not to drink anything until I have published this post. So why are people thirsty? Well on a first grade logic level, it can be deduced that we get thirsty because if we didn't get thirsty, then we would all dry up and die. Our bodies need water to carry on its everyday functions, this I knew previously, but I decided to do some research to find out more.
Your hypothalamus is a part of your brain that, among other things, regulates the amount of water in your blood. When your hypothalamus detects that you bloodstream has too much water, and too little blood (waterlogged) then it tells the pituitary gland to produce less ADH, the hormone that causes more water to be reabsorbed by the kidney.  In case you are wondering, reabsorption is the process by wich the kidney reabsorbs water that it would have lost to urine, so with less ADH comes less water in your blood.  When your hypothalamus detects that the blood is lacking water however, it orders the production of more ADH, conserving your precious remaining water (through reabsorption). 
When your body is lacking water, producing ADH is not enough to save you. Your body needs you to drink some water (or of course ingest some from your food). In order to get you to drink, your body needs to cause a craving and a pain if water is not gained. Your central nervous system (specifically your brain) is where thirst is felt. Your brain becomes aware of your lack of water, and sends your conscious mind the feeling of "If I don't drink water now I will dry up and die!" It is this feeling that truly motivates us to get water, and that really keeps us alive.  Man, I'm thirsty!