There's lots of proof that relationships are good for your health, longevity and life expectancy. Generally that's true, except when it's a bad relationship. 
Stress and other psychological factors have a major impact on our well being. 95% of all illnesses are either caused and/or worsened by stress. Excessive stress contributes to adrenal fatigue which then further contributes to the deterioration of our health.
 
Stress also contributes to an inflammation which can damage our brain cells, it also causes severe depression and other mood disorders. A significant emotional event that is dramatic, unexpected, and stressful can trigger various diseases. Such a sudden shock affects not only the psyche, but impacts at the same time (incredibly this is visible on a brain scan) the part of the brain that corresponds biologically to the specific trauma. Whether the body responds to the unexpected event with a tumor growth (cancer), with tissue degeneration, or with functional loss, is determined by the exact type of conflict shock. Discoveries have been documented and confirmed to support this psychological/physical link. [Dr. Hamer - Germanic New Medicine]

Going through an ugly divorce, dealing with difficult ex-spouses (or children) or psycho friends can tell you in a heartbeat that not all relationships are good for your health. We all have some people in our lives that create stress and problems; some in a major way, some in a minor way. Our relationships in our lives all impact us with positively or negatively. 
 
Researchers have been able to measure relationship quality in a study of 9,000 men and women. After the participants were given surveys about their relationships and the number and type of negative aspects in their close relationships, they were closely monitored for health problems. People with more negative aspects in their close relationships had a 34% increase in the risk of heart problems (even after taking weight, social support and other factors into consideration). Pretty shocking and substantial increase. 
 
Here's the bottom line.
 
Most interaction is good for your health. As humans, we're social animals. Interacting with most of your friends and family members is a good thing. It increases your life expectancy, protects your brain and more. But interacting with some of the questionable friends and family members (we all have them) can actually make you less healthy. Do your best to maximize the first type of interactions while minimizing the second. You can do this by avoiding the people who are negative in your life, using relaxation techniques to help you 
“let go” of stress after encountering these people and learning to actively manage or consciously avoid (if possible) these people so they do not impact your life and your health so much.