If you're mouth dries and you lose desire to continue
Chances are pretty good that you're experiencing mild-dehydration. If your runs last for under an hour, you don't necessarily have to carry a bottle of water with you, or make stops for water breaks. Just make sure that before you head out, hydrate yourself properly with water or a sports drink. Personally, I like to run in the evenings. On my drive home from work, I carry a bottle of water with me to drink along the way. Drinking too much water too close to your runs might discomfort you somewhat, so time them carefully, and the results will speak for themselves.