Good relationships are as important to your health as not smoking, studies consistently show. And for an idea of why that is, a study in Psychological Science linked the positive emotions that come with social contact to the vagus nerve.
Half of the participants practiced “loving kindness” meditation, which focuses on developing feelings of compassion for yourself and others. They were taught phrases like, “May you feel safe, may you feel happy, may you feel healthy, may you live with ease”. The meditators reported feeling happier and more connected to others.
Another reason to be nice: some of the world's oldest people have attributed their longevity to compassion.