Your diet has too much salt in it

Salt pulls water out of and forces the body to conserve as much water as possible, which is why you urinate less when you eat too much salt. The water-deprived cells send a chemical message to the brain asking for more water, and you start to feel excessive thirst. Cut down on your salt intake and make sure you’re drinking enough water.
You went for a morning run

“You’re going to need to drink more on days when you sweat more,” says Dr Peter Maycock. When you exercise, you lose fluids through sweat, and if you don’t replace that fluid, you could end up with excessive thirst. There isn’t one formula as to how much you should be drinking, but Dr Mayock says listen to your body.
You’ve been out in the sun for too long

When summer finally arrives, you spend more time in the great outdoors, whether it’s in a park, on a beach, or in your backyard. Even if you’re not running around, you can still become dehydrated, especially in the hot sunlight. If you know you’re going to be outside all day, make sure you have a water bottle handy.