Kathy Sierra at Creating Passionate Users has written an article on why you should hang out with happy people. With her findings on researches such as mirror neurons, one of the main reasons that she identified is because we tend to imiating others feeling. She also mentions a social science phenomenon called “emotional contagion” and ignorance and misperceptions around the idea of “happy people”:

… But the effect of our innate ability and need to imitate goes way past teenage phone tricks. Spend time with a nervous, anxious person and physiological monitoring would most likely show you mimicking the anxiety and nervousness, in ways that affect your brain and body in a concrete, measurable way. Find yourself in a room full of pissed off people and feel the smile slide right off your face. Listen to people complaining endlessly about work, and you’ll find yourself starting to do the same. How many of us have been horrified to suddenly realize that we’ve spent the last half-hour caught up in a gossip session–despite our strong aversion to gossip? The behavior of others we’re around is nearly irresistible…

Angry/negative people can be bad for your brain – [Creating Passionate Users]