It is practically universal that if someone flashes you a genuine smile, you feel good and are likely to smile back. Remember the last time you started laughing just because other people were laughing? I'm so glad laughter is contagious!
But what I noticed is that even though all emotions can be spread from one person to another, it can be easier to be sensitive to others' pain than to be sensitive to their joy. It seems to take a lot more joy before it starts to rub off on those around you, and it takes very little negativity in comparison before your mood is contagious. If I'm in a good mood, I can be deflated much more quickly and easily compared to how hard it would be to cheer me up when I'm feeling really down.
Of course, some of that depends on our genetic, or inherited, level of emotional resilience, and then we need to take into consideration our own personalities, environments, and the moods we are more habituated toward. (This last part is where therapeutic techniques and practice come into play!) Even when allowances are made for all those factors, it is often true that negative emotions are more contagious than the positive ones. One theory of why this happens is that it is a survival instinct that exists in many animals, not just humans.
If a family, tribe or community is in danger, the first person in the group to notice the threat, is likely to experience worry, fear or anger. If other members of a group are able to tune into the threat and work together to protect themselves, the entire community is likely to be more effective at surviving.
Similarly, it helps to be able to recognize when someone in the group is angry and likely to become violent, giving off the vibe that "nobody better look at me the wrong way today!" If you notice that look on the person's face and recognize the angry body language, this can be fairly effective at telling you to give that person space, thus helping promote survival.
It isn't always a good experience, and not always in our favor. Sometimes people respond to negative emotions by getting angry, especially if negative emotions become chronic and it seems like that person is always depressed or always angry. Maybe I don't want to be around someone who is bringing down my mood, and I blame that person for how I feel when I'm around him or her. Unnecessary conflict often arises from this type of situation. Negative emotions can also become chronic and lead to stress-related diseases, so loving someone with major depression, for example, might put my own health at risk if I am unable to keep my own spirits lifted.
Even complete strangers who are very different from ourselves can trigger feelings of extreme empathy that can be almost intolerable - seeing anyone express anger, tears, frustration, or any kind of discomfort is just not pleasant. If that's true just by yourself, consider how much more difficult that can be when you live in a large family or community of people who pretty much all experience "negative" emotions on a daily basis, like if you work with a lot of negative people. Even if the entire group is happy, and only one person is having a lot of anger or depression, it can ruin it for the entire group! What are the odds that everyone is going to have a good day all at once? Living and working in communities of people doesn't just spread colds and flu. It can also spread all kinds of emotions that are contagious.
Some of us are so sensitive that we don't even know why we are upset or not feeling well. What I have noticed, and I'm glad to see more people are becoming aware about this topic, is that some people are so empathic that they actually feel pain (physical or emotional) that doesn't even belong to themselves. It isn't "just" emotional, such as sadness, not feeling interest in life, feelings of dread and not knowing why, crying spells, panic attacks, irritability and anger outbursts. For many of us, these feelings also come with abdominal distress, headaches, back and neck pain, and all kinds of things that are difficult to know where it is coming from, such as dizziness, numbness or tingling in the extremities, low energy, or feeling faint and tired. We've learned over the years that chronic stress is a major contributor to many major physical illnesses, including cancer and heart disease.
We've all heard of sympathy pains, and can understand that it does not feel good to see someone else suffer, especially someone we love. On occasion, someone will not even be consciously aware the other person was suffering because it was hidden, but relief is noticed when the other person goes away on vacation or is no longer around - sometimes because the other person was a pain, but sometimes just because the other person was in pain. Sometimes people can't tell the difference.
I'm not suggesting you blame others for your emotions, and loneliness and isolation are not what I'm advocating. I'm not telling you to quit your job or end relationships with people you care about. We are social creatures. Even introverts need social experiences to be happy. I don't believe you are "too sensitive" or "overly emotional" if you have high levels of sensitivity and empathy, either. My point is, emotions are contagious and sometimes it might be helpful for us to notice and accept the fact that this is so.
It helps to recognize that if I am feeling very upset or uncomfortable and don't know why, I might be tuning into the feelings or experiences of others without realizing it. Once I realize what is happening, I can then figure out what to do about it. Awareness is an important step in feeling less "out of control" of my emotional experiences. Then I can take care of my own emotional hygiene, the equivalent of washing my hands after using the bathroom to stop the spread of illness.
We all need to take care of our own emotional well being and monitor which thoughts and feelings we are harboring. Each moment we are aware, we can make conscious choices about letting go of negative thinking and negative emotions. Each moment we realize we have a choice, we can feed the positive aspects of our lives and build them stronger. As you become more adept of these skills, you can help prevent yourself from being that person who spreads negativity to your friends, coworkers and loved ones.