To get sciencey for a minute, when you start thinking negative thoughts your amygdala gets hijacked. As shown in the image below, when the thalamus sends stimuli to the amagydala, it gets flooded with that wave of emotion.
The awesome thing is, we can make that wave recede. All we need to do is to think of a happy, positive thought. Our brain can’t be happy and sad at the same time. I love the analogy of a wave, because it’s like our positive thoughts have the power to push the stressed-out, negative thought wave back.