Some common reasons include poor nutrition, lack of exercise, illness and low iron. Another reason that is often forgotten is dehydration. Water is our primary life source. It helps our blood flow, cleanses toxins from our body and keeps our skin healthy.
The average woman needs 2.7L of fluid/day and men need 3.7L. Keep in mind this is total fluid. It includes fluids from milk, coffee, tea, juice, soup, fruits and vegetables. So if you have a diet rich in these things you only need to make up the difference in water.
If you wait until you are thirsty before you drink, realize that you are already dehydrated at this point. You can also use your pee as a reference. Light yellow color is a sign of good hydration (keep in mind medications and supplements can change the color).
So if you feel tired, suffer from dry or acne prone skin or even headaches, a few glasses of water may help. Keep in mind your water intake needs to be even higher when it's hot out or you exercise.