Edited excerpt from Complaining Is Terrible for You According to Science by Jessica Stillman:
Not only do repeated negative thoughts make it easier to think yet more negative thoughts, they also make it more likely that negative thoughts will occur to you just randomly walking down the street. Being consistently negative starts to push your personality towards the negative.
Hanging out with negative people does much the same. When we see someone experiencing an emotion (be it anger, sadness, happiness, etc), our brain ‘tries out’ that same emotion to imagine what the other person is going through. And it does this by attempting to fire the same synapses in your own brain so that you can attempt to relate to the emotion you’re observing.
“When your brain is firing off these synapses of anger, you’re weakening your immune system, raising your blood pressure, increasing your risk of heart disease, obesity and diabetes, and a plethora of other negative ailments,” Psych Pedia author Steven Parton says.
The culprit is the stress hormone cortisol. When you’re negative, you release it, and elevated levels of the stuff, “interfere with learning and memory, lower immune function and bone density, increase weight gain, blood pressure, cholesterol, and heart disease.”
(1) Cf. “I think the mind is like a forest floor: the more you walk paths the deeper they get and the easier it is to walk them again.” — from You can train yourself to think positively.
(2) Cf. Ideas spread inside a company due to positive energy; 8 ways to increase it.