WebMD has the skinny on why friends contribute to your health and well-being:
"People with social support have fewer cardiovascular problems and immune problems, and lower levels of cortisol -- a stress hormone," says Tasha R. Howe, PhD, associate professor of psychology at Humboldt State University. "Why? The evolutionary argument maintains that humans are social animals, and we have evolved to be in groups. We have always needed others for our survival. It's in our genes. Therefore, people with social connections feel more relaxed and at peace, which is related to better health."
For more details, read the full article.