A person who is chronically negative, by definition, is one that is habitually negative. When someone forms a habit, it moves from the part of their brain that is self-aware (the frontal lobes) to deeper parts of their brain that are unconscious and automatic. The more anyone does anything, the less aware they are of it. Thus, negative people are usually not aware of how negative they are.
We have to be very deliberate to become aware of our own negativity. The good news is that as we become aware of it, our brains become less negative.
NEURO-EXERCISE:Can you can go an hour without a negative thought or feeling? Give it a shot. This includes worries, doubts, fears, disappointments. When you have a negative thought, write it down. It will help you become more aware. Don't be surprised if you have as many as 25 negative thoughts or feelings an hour. The more you do this exercise, the more you'll see your negativity drop. For even faster progress, enlist your brain's reward circuitry by promising yourself an indulgence if your negativity substantially drops. This exercise will significantly improve your brain's performance and enrich your experience of life.