Staying out in the sun all day or eating a spicy food is normal for you to drink water, but if you are always thirsty or if you drink as much water as you can, nothing will quench your thirst, it may be more than temporary dehydration.
Dry mouth
Logically, when you feel your mouth as "dry", it will make you thirsty. The dry mouth condition is otherwise known as xerostomia and in this case, your glands do not produce enough saliva to keep your mouth moist. You can refer this problem to your doctor / dentist.
dIABETES
When you have diabetes, glucose builds up in the blood, forcing the kidneys to work hard to absorb it. When the kidneys can no longer support you, you will produce more urine than normal. Frequent urination will bring thirst and this will cause the body to require more fluids / water.
Anemia
As anemia worsens, it can cause thirst along with typical symptoms like weakness and fatigue. The body loses red blood cells faster than it can replace them and will try to compensate for fluid loss by causing thirst.
Low carb diet
If you are on a low carb diet, like the popular diet keto, you may find that you are more thirsty than usual. It turns out that this is a normal side effect.
More exercise
Another reason why you are more eager is simply because you have increased the intensity of your workout. If you are more active than before, the amount of water that has worked for your body before may not be enough. When your body sweats more, it is losing more fluids and you will need to start drinking more to compensate.
Source: Men's Health
