Negative thinking is a Poor Habit.
When you make negative thinking a habit, negative things eventually will materialize in your life.
The brain, specifically the subconscious (Brain Stem and Limbic System), goes to work behind the scenes to bring about that which you are constantly thinking about.
It does this through intuition. Intuition is the way the conscious brain (Neo-Cortex) communicates to you what the subconscious wants you to do. If your thoughts are negative, the intuition you will receive will be destructive. It will nudge you to do something that will bring about the negative things you are constantly thinking about. When you are in a negative place, intuition can be destructive.
This is why it is so important to be aware of our thinking and stop all negative thoughts in their tracts. This is also why it is so important to consciously put positive thoughts in our minds through the use of daily affirmations and visualization techniques. The conscious mind cannot hold onto two thoughts at the same time. so, the key to getting rid of negative thoughts is to think about something positive. This pushes the negative thought out of the mind and prevents the subconscious from doing any damage.