Negative thoughts drain you of energy and keep you from being in the present moment. The more you give in to your negative thoughts, the stronger they become. I like the imagery of a small ball rolling along the ground, and as it rolls, it becomes bigger and faster.
That's what one small negative thought can turn into: a huge, speeding ball of ugliness. On the contrary, a small positive thought can have the same effect blossoming into a beautiful outcome.