If you're thirsty then you're already dehydrated. So drink lots of fresh clean water. And I mean water. Soda's, coffee, tea, and other sorts of drinks you can purchase are not the same as water. They are treated as foods by the body.
Water is one of the best purifiers for the body and a necessary ingredient for life.
The second reason is lack of exercise. If you don't move neither will your bowel. So move everyday. Walk 1 hour every day and see how much more energy you will have and how much better your bowel will move. Third is too much junk food and not enough fiber.